Hi Codeforces!

Meet a small innovation on Codeforces — difficulties of problems (and at the same time a new widget filtering problems in the archive). For all the problems of the archive, I’ve calculated the difficulties in the scale of the rating of participants. Approximately this means that if the rating of the problem is equal to yours, then on a typical round you would solve the problem with a probability of 0.5. And, in general, if your rating is r_i, and the problem rating is r_j, then the problem during the round can be solved approximately with probability:

For example, if the rating of a problem is less than yours by 200, then the expected probability of solving the problem is 0.75. With a difference of 400 rating points, the probability increases to 0.9.

For convenient search of problems in the archive, you can now use a special widget:

With it, you can find not only problems that have all the chosen tags, but also which have at least one tag from the list.

A difficulty of a problem is also displayed when choosing problems in a mashup.

I hope that now you will be able to do more effective practice, and the process of making new mashups for trainings will become easier.

UPD 1: Have you already noticed new pop-ups about judgment verdicts of your submissions?

UPD 2: (API update) Optional field rating has been added to the object Problem in API.

UPDATE: The form for team registration appeared. Data will be transfered to ICPC database. Teams who filled similar form for the fixed site, do not need to fill this form twice..

Teams Registration

Coordinators of sites, allowing only teams from host university already received the links to similar forms with the pre-selected site.

The 2018 ICPC Uzbekistan Subregional Contest will take place in multi-host mode at Sunday, November 11, 2018. The universities may register as sites of the 2019 ICPC Uzbekistan Subregional using this form.

Register the Site

The form for teams registration will be posted later.

P.S. Breaking news: the hosts from Tajikistan and Turkmenistan are welcome to register and participate in Uzbekistan Multi-Site Subregional. Teams from those two countries — time to join ICPC now!

I would like to invite you to participate in the rated Codeforces Round 519 by Botan Investments. Date and time of the round: Oct/28/2018 18:35 (Moscow time).

This is a combined round having 7 problems and lasting 2 hours, and it will be rated.

Tasks for you were prepared by Anadi, Grzmot, isaf27 and Rzepa. Also thanks for:

KAN and cdkrot for help in preparing the problems; --Pavel--, Nerevar, map, GR1n, rutsh, AlexFetisov and winger for testing the round; MikeMirzayanov for Codeforces and Polygon platforms.

The round is supported by Botan Investments.

Prizes! Top 50 participants and 20 random participants with rank from 51 to 500 will receive personalized hoodie with CF handle.

Scoring distribution will be announced later. I wish you a high rating and looking forward to see you at the competition!

UPD: I'll be on the community Discord server shortly after the contest to discuss the problems.

UPD: Scoring: 500 1000 1500 2000 2250 2750 3500

UPD: Editorial

The round is over, congratulations to the winners!

1. scott_wu
2. mnbvmar
3. HIR180
4. ksun48
5. Benq
6. geniucos
7. Alex_2oo8
8. Petr
9. Um_nik
10. V--o_o--V

The Moscow Institute of Physics and Technology welcomes high school students to take part in the programming boot camp Moscow Workshops Juniors! The boot camp will be held from February, 25 to March, 6.

The educational program includes daily studying contests, thematic lectures and practice. In a free time participants could join the popular science lectures, do sports, play the intellectual games and be involved in other activities. According to the results of the introductory contest the participants are divided into divisions depending on their skill level. Participants can choose the most interesting topics and make an appropriate training program. If a participant finds the curriculum too simple or difficult, he can change division.

To take part in the camp, you have to pass qualifying online tours, its format is close to the Olympiad in Informatics. You can participate in several rounds or only in one tour, the ranking of the participants is made according to the results of four rounds. The selection rules can be found by the link.

There are four qualifying rounds:

October, 27 at 16:00 pm UTC+03

November, 18 at 12:00 pm UTC+03

December, 16 at 12:00 pm UTC+03

January, 20 at 12:00 pm UTC+03

Moscow Workshops Juniors suggests awarded high school students several opportunities of participation. If you are a prize-winner of national Olympiads in informatics 2017-2018, you are invited to take part in the Workshops free of charge! For participants of national Olympiads in informatics 2017-2018 the Workshops cost $400, for another participants – $700

REGISTRATION

Have questions still? Write to organizing committee email: [email protected]

On Oct/25/2018 17:35 (Moscow time) Educational Codeforces Round 53 (Rated for Div. 2) will start.

Series of Educational Rounds continue being held as Harbour.Space University initiative! You can read the details about the cooperation between Harbour.Space University and Codeforces in the blog post.

This round will be rated for the participants with rating lower than 2100. It will be held on extented ACM ICPC rules. The penalty for each incorrect submission until the submission with a full solution is 10 minutes. After the end of the contest you will have 12 hours to hack any solution you want. You will have access to copy any solution and test it locally.

You will be given 7 problems and 2 hours to solve them.

The problems were invented and prepared by Roman Roms Glazov, Adilbek adedalic Dalabaev, Vladimir vovuh Petrov, Ivan BledDest Androsov and me.

Good luck to all participants!

Our friends at Harbour.Space also have a message for you:

Hey Codeforces! We want to remind you that the Scholarship for the Master’s in Robotics programme, which starts on January 7th 2019, has an application deadline of November 12th, 2018.

Harbour.Space University and Remy Robotics are collaborating to offer graduate students from anywhere in the world a once in a lifetime opportunity, a fully funded scholarship for Harbour.Space University’s Master’s Programme in Robotics.

The scholarship value is €34.900 and it includes:

• Complete coverage of the University tuition fee (€22,900)

• Internship at Remy Robotics (20h per week during 1 year)

• €1,000 per month during 1 year (Internship earnings)

UPD: vovuh and me will be waiting for you on the community Discord server shortly after the contest to discuss the problems.

Congratulations to the winners:

Congratulations to the best hackers:

And finally people who were the first to solve each problem:

UPD2: Editorial is published

part 2: https://codeforces.me/blog/entry/62792

Watch my lecture-stream tomorrow (Thursday) at 14:00 CEST — https://www.youtube.com/watch?v=qdlPY37MBPo https://www.youtube.com/watch?v=U_h3IjreRek. I will go through theory and problems from this blog. The only prerequisite is knowing what is probability. The next (harder) part on Monday.

The video will be available later, with timestamps for each problem — so you don't have to watch everything.

Definition of EV

Let's say we bought a lottery ticket for 2$. We will win 10$ with probability 10%, and 20$ with p-bility 2%. On average, it gives us 0.1·10 + 0.02·20 = 1.4, so we are worse off after buying the ticket. The computed average is called the expected value.

The expected value (EV, expectation) is the average value of an event/experiment. For example, EV of the number of pips rolled on a 6-sided die is 3.5:

Linearity of EV (super important theorem):

I would like to invite you to participate in the rated Codeforces Round #518. Date and time of the round: Wednesday, October 24, 2018 at 18:05. The round was postponed from October 23 to October 24 because of the ICPC Indian Online Qualifier.

This is the first competition I proposed. I hope you will enjoy it.

The round will have 6 tasks for the second division and 5 tasks for first (3 problems are common). The contest will last for 2 hours.

Tasks for you were prepared by Alexey kristevalex Kristev and Alexey Um_nik Danilyuk. Also thanks for:

Nikolay KAN Kalinin and Ildar 300iq Gainullin for help in preparing the problems; Ivan isaf27 Safonov и Oleg Merkurev Merkurev for testing the round; Mike MikeMirzayanov Mirzayanov for Codeforces and Polygon platforms.

I would like to express a special gratitude to Um_nik for the help throughout the process of preparing the round and also for patience for my stupid questions.

This round is a thanks-round for support from Mail.Ru during the 8-year campaign!

Scoring distribution will be announced later. I wish you a high rating and looking forward to see you at the competition!

I'll be on the community Discord server shortly after the contest to discuss the problems.

UPD: scoring distribution div1: 500 1000 1750 2250 2500 div2: 500 1250 1500 2250 2750 3500

UPD2: edtorial.

Hello, codeforces!

Sorry for the long break, but the last weeks of holidays and the first weeks of academic year took my attention. I hope today's trick will make you forgive me. :P

I invented this trick a few years ago, but for sure I wasn't first, and some of you already know it. Let's consider the following interactive task. There are n (1 ≤ n ≤ 10⁵) hidden integers a_i, each of them from range [1, 10¹⁸]. You are allowed to ask at most 103000 queries. In one query you can choose two integers x and y (1 ≤ x ≤ n, 1 ≤ y ≤ 10¹⁸) and ask a question ''Is a_x ≥ y?'' The task is to find the value of the greatest element in the hidden array. The checker isn't adaptive.

Unfortunately, this task is only theoretical, and you cannot solve it anywhere, but it'll turn out, that solution can be handy in many other, much more complicated problems.