Hi.

Of course, there was an unpleasant incident with the problem today. In fact, the problem almost coincided with another problem in the old round, which was hosted 5 years ago.

You, of course, are shocked. You, of course, think that the round should be unrated.

You are wrong. Here's why.

Hello, Codeforces.

It's time to take stock of last year. Shall we begin? And we are happy to sum up the results of the last season of Global Rounds. We announce the results!

We remind you that the global rounds are a joint initiative of XTX and Codeforces.

We hold 6 such rounds per year. All of them are open to both divisions. At each such round, 50 brand T-shirts were handed out, and we are happy to give T-shirts to all problem authors.

The prizes for the 6-round series are:

• In each round top-100 participants get points according to the table.
• The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
• The best 20 participants over all series get sweatshirts and place certificates.

Hello, Codeforces!

The year 2022 is finally coming to an end. Let's be clear. It's been a terrible year. Tomorrow I will make a wish, and you can easily guess which one.

I am writing these lines, and in a few minutes the Good Bye 2022 round will begin. And this inspires optimism and even pride in the community. Despite all these difficulties and troubles, we make rounds together and participate in them. Let's be grateful. And for this, let's say symbolic thanks to 2022 and a lot assistants: coordinators, problem writers, testers, participants and sponsors. We are many and we are together.

Can I have a little more detail on the Codeforces team?

Let me separately say thanks and words of admiration for the work of the coordinator of coordinators KAN and the senior developer geranazavr555. Both of you bring a lot to Codeforces, but your activity is not always noticeable. We appreciate it. Thank you!

I send thanks to Una_Shem: in the difficult conditions of broken global logistics, you do a lot so that the gifts are delivered to the winners, and the authors receive their reward.

Thanks to unreal.eugene for contributing to the development and kuviman for helping with setting up ci and other pipelines.

Who I will not say thank you to, it's cheaters! Ugh, to be like that. Please stay in 2022.

I'd like 2023 to be the year we regret leaving exactly one year from now.

Please make us happy!

Make our wishes come true.

* Image by ligrenok.

Dear Santa Claus. Please make 2023 a happy year, not just that. Please, we all need this.

Hello, 2023!

And we have traditional gifts!

Change Handle Feature

Hurry! Only until the 10th of January, you can change your handle (but only once)! Note that it will be possible to roll back the changes or change the handle again only after a year. Be careful what you wish for.

You can change your handle to the new one which wasn't used before by anybody or which was used by you before. The links to a profile page with an old handle would automatically redirect to the actual profile.

Again, this year if you took part in at least 10 rounds you can request a handle of an inactive participant. It means that the participant should have a period of activity on Codeforces of at most 180 days, this period should be in 2020 or earlier. The inactive participant can't have posted comments, messages, and so on. It can't take part in more than 2 contests. It will be automatically renamed and informed by email. A user has the opportunity to request back his/her handle: in this case, we will roll back the change and return your previous handle to you. If you can't change your handle to another, it means that some requirements don't meet. Please do not ask me to do something with it. Especially, I do not like requests like "this is my second account, I took an interesting handle myself". I'm not Santa Claus.

Talking about handles I always
reminisce the following story. Once a user wrote me the message: "Please change my handle from I_love_Valya to I_love_Sveta, as I no longer love Valya ..."

New Year's Masquerade of Colors and Ranks

The traditional magical tab has appeared in the profile setting. Happy New Year!

Hello Psyho, this is Mike. We are happy to help with likes. So, Codeforces?

He waits for your likes here: https://twitter.com/FakePsyho/status/1605570944537280512?t=MV0OksdeUb8FXKApncDZcw&s=19

I believe that 40 facts are too few for us. We need more!

Hello!

Tomorrow (December 7th) the ICPC Northern Eurasia Finals (previously known as NEERC) will take place. The competition will be held at several venues: St. Petersburg, Barnaul, Kutaisi and Almaty. Almost 300 teams will take part in it.

We invite you to join the online mirror of the competition: 2022-2023 ICPC, NERC, Northern Eurasia Onsite (Unrated, Online Mirror, ICPC Rules, Teams Preferred). It will start at Dec/07/2022 11:05 (Moscow time). We recommend participating in teams, but experienced individual participants will also have fun.

The duration of the competition will be 5 hours. Of course, the round is unrated.

Good luck!

Hi.

Sometimes it seems like all you have to do is change a couple of inscriptions on the site and that's it. But it's not that simple.

For example, I decided to change my friends to followers/subscribers. In fact, the relation we have here is more like a following or a subscription than a friendship.

But there's a tiny issue.

Sometimes this relationship is used in the interface in one direction (we are talking about those who follow us). Then everything is easy in this case, we can mention them as your followers.

Sometimes, it's about those you're following. Then a little more difficult. The best thing that I came up with and advised me is the term "following". But replacing "Friends Standings" with "Following Standings" (or "Followings Standings"?) looks so-so. Likewise, what was "Friends Rating Changes" and will it be "Following Rating Changes" (or "Followings Rating Changes")? Looks really clumsy.

Probably, you can change the interface so that to get rid of these menu items at all. But I don't want to do that now. It would be wonderful to find some way of writing that would be most successful.

Thanks to Ivan and Peter, with whom I have discussed this problem a little, but I haven't found a good solution.

What are your thoughts?

Hello, Codeforces.

Look what an interesting thing I've been working on the last few days. Now, if a problem uses test cases and its validator is written in the recommended way (using setTestCase), then the example inputs are displayed using different colors.

Just follow the link and see how beautiful it turned out: https://codeforces.me/contest/1714/problems!

So far I have enabled this only for the last few rounds, but if everything works as intended, I will enable it for more rounds.

In order to support this, I had to tinker a lot. I made changes to testlib.h so that the validator can return test markup. Support for this markup required changes in various system services. But I think it's worth it!

The coolest thing is that it was possible to implement all this without changing existing problems. If a validator is used using the setTestCase feature, then this improvement can be applied to this problem.

Of course, it would be cool to have similar highlighting in example outputs as well. And even, say, synchronously highlight an input-output test case pair when hovering over. Unfortunately, standard checkers are often used, in which information about test cases is lost. Perhaps I'll apply some heuristics to make it work in most cases.

Hello, Codeforces.

I have wanted for a long time and now, finally, I sat down and did it.

Many of you are familiar with my Testlib project. This is a library that helps you develop problems if you are using C++. In the case of problems for Codeforces rounds, its use is mandatory. It seems to me that it is almost a de facto standard for preparing materials for completely different olympiads and competitions.

The history of С++-Testlib is almost lost in centuries, I published the first version in 2005.

Nowadays, making edits to Testlib has become tricky. The code grown to thousands of lines, also it has too long a history. This increases the likelihood of accidentally changing some aspect of behavior, which will jeopardize the holding of a Codeforces round or another important competition. In short, making a bug in this code or changing behavior anywhere is highly discouraged.

So I recently took the plunge and wrote some tests for Testlib. Here's what tests can do now:

• check that all cpp files in the repository are compiled (test test-000_compile-all-cpp)
• check the plausibility of the behavior of a simple sval.cpp validator on a series of files (test-001_run-sval test)
• check the plausibility of the behavior of the two checkers fcmp.cpp and wcmp.cpp on a series of files (test test-002_run-fcmp-wcmp)
• check that the behavior of the random number generator has not changed (test test-003_run-rnd)
• check the behavior of some functions and the behavior of InStream and opt in a series of unit tests (test test-004_use-test.h)

More good news. All this was automated through GitHub Actions. Tests are automatically run on any push or pull request.

Hello, Codeforces.

We have exciting news! Look at the logo. See the changes?

Yes, we are happy to announce our new sponsor and partner – the TON Foundation!

We appreciate their support!

The Open Network (TON) is a fully decentralized layer-1 blockchain designed by Telegram to onboard billions of users. The TON Foundation is aimed to further develop and advance the TON Blockchain and its related products for a mass audience.

Andrew Rogozov, founding member at the TON Foundation, says, "We strongly believe in the idea of competitive programming itself, because it is in the TON Blockchain DNA." The TON Blockchain protocol was designed by Nikolai Durov — who is a two-time ICPC world champion, a three-time IMO gold medalist, a multiple IOI medalist, and a co-founder of Telegram — and other winners of international competitions. Now the TON Blockchain core team consists of winners of blockchain contests held by Telegram.

The TON Foundation has already supported the CodeTON Round 1 earlier in March 2022. More than 22K users were registered, and 12K participants made at least one submission. "Codeforces is an unparalleled platform for developing the programming skills of coders all over the world. We are of one mind with the Codeforces team that such contests reveal a treasury full of talents," adds Andrew Rogozov.

I send rays of gratitude to Telegram and personally to Pavel Durov, who has supported our community for many years. With this help we have come a long way — we have run hundreds of rounds, developed the ecosystem, our community has grown hundreds of times. And we are moving forward! Support from the TON Foundation pleases with its succession.

I'm happy to announce that starting July 2022, we have the option to increase problem writer payouts. Hooray!

• Div. 2: $300;
• Div.1 + Div. 2: $600;
• sponsored round: $900.

In addition, we continue to encourage payments in cryptocurrency. In this case, we pay out +25%. I look forward to new rounds on our platform.

— Mike Mirzayanov

Hello!

I always like to think that Codeforces is not only a place where you come to compete but also a place where you can learn. Learn new things.

Two years ago, we launched the EDU subproject, a platform for publishing classes. There are already several classes, but there are so many interesting topics around!

The Codeforces team can write the platform, make the technical foundation, but it would be great to write content with the support of the community.

Thanks to the support of competitive programming alumni, we have a budget for targeted grants for the preparation of new classes in the EDU section. Hope we have new content soon!

We are looking for lesson authors who could prepare and record one or more lessons. Those who are now red or have ever been red are invited.

We assume that one lesson (class) consists of:

• 3-5 steps: break the lesson into logical parts,
• video materials explaining the topic (usually, the total duration is 60-120 minutes),
• text summary
• a thematic training set of problems with a wide range of complexity (from the most basic on this topic to quite tricky ones)
• first of all, we would like to cover the main topics of competitive programming — DP, graphs, basic data structures, etc.

Please note that we expect you to record and edit the video by yourself. Look at already recorded lessons to understand the expected format and structure of lessons. We have a rich history of teaching, so you can count on our advice and assistance in preparing lecture plans and problems. If video editing scares you, then here we can help.

We invite you to help grow the community and make history. I am sure that the recorded classes will be watched by tens of thousands of people (or more?).

The reward for recording one class is $1000.

If you are now red or have ever been red and you are interested in this offer, then fill out the form (open until June 28th).

Hello, Codeforces!

Recently, we have repeatedly encountered the fact that community members get confused in rounds, mistaking unofficial training in mashups with a round, which is prepared by the Codeforces team and the writers together with our coordinators.

To make it a little clearer, I ask you not to use "Codeforces Round ..." in the names of unofficial training rounds (mashups). Probably, if you are doing a series of such rounds, then you better come up with your own naming — this will make it clearer to the audience.

P.S. If you see that any author has not noticed my appeal, then please let him know.

Thank you!

Hello.

In the meantime, the onsite event has already begun. You can follow the results at the link https://nerc.itmo.ru/archive/2021/standings.html (refrain from viewing if you want to plan to write a mirror and want the conditions as close as possible to the participants in the competition).

There is great news. This year it was possible to get together without any online participation. Teams write from one computer! Good old ICPC.

And I suggest you join the online mirror. It is designed for team participation by those who have passed the qualifying competitions. Ready to try? Use the link: 2021-2022 ICPC, NERC, Northern Eurasia Onsite (Unrated, Online Mirror, ICPC Rules, Teams Preferred).

Good luck!

Hello.

I started processing responses on the form where we collected your TON wallet addresses. Unfortunately, many winners did not complete the form. I contacted some of them in PM and it turned out that this was not intentional.

We are extending the deadline for completing the form until 2022-04-06 23:59:59 (UTC +3). Please, if you want to receive coins, then fill out the form. A valid address looks like a random 48-character string. To register a wallet, follow one of these links: https://tonkeeper.com/ or https://wallet.ton.org/

The form can be filled out at https://codeforces.me/userForm/203c74605996e40f

Coins will be received by those who are at the top-1000. Additionally, those who solved at least one problem in this round will receive 1 coin each. Details can be read at the link.

The form will be processed on April 7th. And in a week after it, we will send out coins. Please do not disturb me or the organizers with questions that are answered in this post.

Also I remind you about TON Smart Challenge contest aimed at contract source code optimization: https://t.me/toncontests/26 It's not too late to connect and try your hand

You can read more about this competition here:

Hello, Codeforces.

We live in difficult times. And, unfortunately, our community is highly dependent on world events around us. Three years ago, it seemed easy to send a t-shirt anywhere in the world. Now it's a quest with many unknowns. Just a few months ago, it was not difficult to send a reward to the problem writer. But now, we are cut off from the outside world and regulated by multiple new laws.

And here it is very important not to lose, not to break what has already been done. Don't stand still, move forward. I am very glad and grateful to the problem writers and coordinators that in March we managed to hold 10 ratings (we held 9, come to participate in the tenth). Thanks to everyone who took part in the preparation!

And I want to share another great news. Our friends NEAR has come up with the initiative to support the community during this difficult time. We are pleased to announce that the upcoming rounds will be funded by NEAR, which will help with the payment of rewards to problem writers and coordinators. This is a timely and important help. Thanks!

It will be easier and faster for us if the problem writers or coordinators have the opportunity to accept a reward in the form of a cryptocurrency transfer. In this case, we are ready to transfer 25% more and make the transfer much faster. Oh, it's a pity we can't send t-shirts with crypto.

And I remind you that the next round (and Codeforces Round #780 (Div. 3)) is waiting for you) will take place very soon, on Mar/31/2022 17:35 (Moscow time). I hope to see you among the participants.

— Mike

TLTR: If you are in the top-1000 of CodeTON Round 1, then fill out the form by the link https://codeforces.me/userForm/203c74605996e40f. Also, anyone who solved at least one problem in the round can fill out it and get 1 TON airdrop. The deadline is 23:59:59 (UTC +3) 04.04.22.

Hello. Great news from the TON Team. I give them the floor.

Good news everyone!

We decided to celebrate the successful finish of the CodeTON round by airdropping all the people who have solved at least one of the problems (officially, practice and virtual participation don't count) of the CodeTON round. Such participants are eligible for 1 TON airdrop and a memorable NFT. Please do note that such NFT bears only symbolic value and is not entered to have any financial or investment value.

Please submit your TON wallet address via the link https://codeforces.me/userForm/203c74605996e40f if you are eligible for such an airdrop by 23:59:59 (UTC +3) 04.04.22.

Winners are more than welcome to provide organizers with the TON Wallet addresses to claim their prizes.

We also want to invite all of you to try yourself at TON Smart Challenge contest aimed at contract source code optimization: https://t.me/toncontests/26

You can read more about this competition here:

Thank you for taking your part, hope you enjoyed the round, looking forward to meeting you later!

TON Foundation

Tomorrow will be the first global round of this year. And we are happy to sum up the results of last season. We announce the results!

We remind you that the global rounds are a joint initiative of XTX and Codeforces. We are happy to announce that they continue into the new year.

We hold 6 such rounds per year. All of them are open to both divisions. At each such round, 50 brand T-shirts were handed out, and we are happy to give T-shirts to all problem authors.

The prizes for the 6-round series are:

• In each round top-100 participants get points according to the table.
• The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
• The best 20 participants over all series get sweatshirts and place certificates.

The final standings after six rounds (sorted by total points in four best placed rounds):

Hello, Codeforces.

Unfortunately, we have to declare the round unrated.

Absolutely, these are not issues of the side of Codeforces code, but rather it is a hardware issue. Probably, electricity, power unit or something else. This led to unexpected repeated reboots of the server with the database. I hope that we will be able to quickly deal with this.

We apologize to the writers of the round and its participants.

P.S. Please, do not downvote the round announcement: the writers are not to blame for what happened. And in general, this time it is difficult to blame someone. Sometimes hardware fails.

Mike.

But why not!

Many people here seem to know me. Or maybe not quite? I am ready to answer your questions. Ask me anything! I plan to spend no more than 3 hours on answers. Sorry if I can't answer all the questions.

Let's more or less follow the rules from here https://www.reddit.com/r/IAmA/wiki/index

Below is actually a quote from the rules.

The interviewee begins the process by starting a post, describing who they are and what they do. Then commenters from across the internet leave questions and can vote on other questions according to which they would like to see answered.

The interviewee can go through and reply to the questions they find interesting and easily see those questions the internet is dying to have the answer to.

Because the internet is asking the questions, they're going to be a mix of serious and lighthearted, and you'll find yourself sharing all kinds of things you won't find in a normal interview.

Comments will be removed under a few circumstances:

• Abusive or harassing comments.
• Requests for personal favors from the OP (For example, "OP, can you send me a signed autograph").
• Top-level comments must ask a question.
• Comments where there would be no possibility of a real answer, especially where it is deliberately creepy or offensive.
• "I bet OP won't answer this"-type responses, which usually come after the OP has finished responding to questions.
• "Fluff," non-contributing responses from users, responding to all of the OP's comments for karma/attention.
• Repeatedly asking the same question.

Please, don't attempt to bypass the rules by adding a ? to a nonquestion.

Questions must be directed toward the individual(s) doing the IAMA.

Under our policies, astroturfing is the practice of an individual or group of individuals who plant questions in an IAmA post for a particular purpose. This kind of behavior is forbidden.

UPD 1: Oh, I'm kind of tired of doing this. Please keep asking questions. Tomorrow I will answer again. Don't forget to vote for the most interesting ones. I will not have time to answer everything, I will choose something. Thanks!

UPD 2:

Hi.

Here is the list of top-voted blog posts in 2021. Find something interesting for yourself?

Hey!

Are you celebrating the New Year already? I wish you a Happy New Year. I wish you progress, joy and health.

We decided to show the current progress in the catalog of significant Codeforces posts.

So far, quite a bit has been implemented, but somehow you can already use it! Would you like to see it? Follow the link:

As planned, the catalog will collect and classify interesting and useful posts from Codeforces. Indeed, many useful articles have been written in Codeforces over 12 years (God, already 12 years!).

The catalog will be a kind of Codeforces golden fund in the future.

The catalog is a tree of topics to which posts can be added. We will still work on its functionality, for now there are only basic features. Don't be discouraged if something is not supported, which is very necessary — just let me know!

Any red user can enable directory editing. Remember that it is extremely important to make all changes thoughtfully and carefully. The tree should not become infinite in size. You need to have a sense of proportion when detailing topics. Please add only good, useful articles to the directory. If there is a disagreement on the contents of the catalog, the administration will make the final decision.

Hope this will be helpful. And together we can put together a catalog and keep the catalog up to date.

P.S. Many thanks to KAN who helped a lot with suggestions and catalog initial content.

Hello.

Here's a small improvement for posts and comments. Likes can now be specified in their markup. Maybe this will be useful for organizing some kind of polls or collecting feedback.

Here are some examples:

Yes, I know that this feature can be improved. I would prefer that we try to use it a little and understand what exactly is missing the most. Maybe we need to implement the fully functional polls.

Let's do a small survey using the new feature.

1. What is your age range?

   • under 16:
   • 16-20:
   • 21-25:
   • 26-35:
   • 36+:

2. Tabs or spaces?

   • tabs:
   • spaces:

3. Have you had covid??

   • yes:
   • no:
   • I don't know:

4. Lately, there have been a lot of meaningless blogs on Codeforces. Is it worth strengthening measures to fight against them?

   • yes definitely:
   • rather yes, but not necessarily:
   • rather no, I read some of them:
   • no definitely:

Hello Codeforces!

Oh yes, 2021 ends! I recently realized that 2022 is pronounced like "2020 too". Not that. I want a completely different year!

Anyway, we did a nice job this year. I hope we have pleased you with interesting problems. Thanks to the round coordinators and writers! You make our community richer. Please make us happy in 2022!

I wish you all interesting tasks and brilliant solutions! I wish you error-free code and wish us only rated rounds :-)

Hello, 2022!

And we have traditional gifts!

Change Handle Feature

Hurry! Only until the 10th of January, you can change your handle (but only once)! Note that it will be possible to roll back the changes or change the handle again only after a year. Be careful what you wish for.

You can change your handle to the new one which wasn't used before by anybody or which was used by you before. The links to a profile page with an old handle would automatically redirect to the actual profile.

Again, this year if you took part in at least 10 rounds you can request a handle of an inactive participant. It means that the participant should have a period of activity on Codeforces of at most 180 days, this period should be in 2019 or earlier. The inactive participant can't have posted comments, messages, and so on. It can't take part in more than 2 contests. It will be automatically renamed and informed by email. A user has the opportunity to request back his/her handle: in this case, we will roll back the change and return your previous handle to you. If you can't change your handle to another, it means that some requirements don't meet. Please do not ask me to do something with it. Especially, I do not like requests like "this is my second account, I took an interesting handle myself". I'm not Santa Claus.

Talking about handles I always
reminisce the following story. Once a user wrote me the message: "Please change my handle from I_love_Valya to I_love_Sveta, as I no longer love Valya ..."

New Year's Masquerade of Colors and Ranks

The traditional magical tab has appeared in the profile setting. Happy New Year!

In 2021 (as in 2019 and 2020), with the support of XTX Markets, 6 rounds of the new Codeforces Global Rounds are held: common rounds for both divisions of 7–9 problems each. The duration of the rounds is 2-3 hours, depending on the number and complexity of the problems. All such rounds are rated for all participants. At each such round, 50 brand T-shirts are handed out, and we are happy to give T-shirts to all problem authors.

The prizes for the 6-round series in 2021:

• In each round top-100 participants get points according to the table.
• The final result for each participant is equal to the sum of points he gets in the five rounds he placed the highest.
• The best 20 participants over all series get sweatshirts and place certificates.

Current standings after five rounds (sorted by total points in four best placed rounds):