Hello.

This round will start Yandex.Algorithm 2011. The problems of the round were prepared by me, of course, with the help of the Codeforces team and Yandex.

I hope you enjoy the problems and their solution will start a successful performance at the tournament.

As you have already noticed — the system operates in a somewhat truncated form. We decided to run it in safe mode and turn off some functionality at the time of the contest. After the round everything will be back.

I recall that the top 500 participants will receive a ticket to the first online round of the Yandex.Algorithm. However, if you do not get to qualify at this time, do not despair — you can participate in the second qualification, which will be held on May 6 at 15:00 (UTC).

I wish you have a fun,
MikeMirzayanov

Hello!

As you have already noticed the menu will link to the results of the competition has appeared. Drum roll, fanfare, applause, and the blinding spotlight! We congratulate the winners:

• Winner is Aleksey, 33 score points!
• The second place — mrButt, 30 points
• The third place — Sanya_M, 29 points
• The fourth place — Ruslan, 25 points
• The fifth place — MaxBuzz, 24 points

Congratulations to all winners! Note that the program by Aleksey has surpassed all expectations of the jury erred solely on hard-to-recognize archives.

I'm asking all winners to send me a private message, we have something to discuss.

MikeMirzayanov

UPD: One test has been found in which the author's answer was incorrect. I've rejudged solutions, but the order of participants has not changed :)

The coding phase has been finished. Please wait for the final testing and results.

Good day everybody.

Everything written below is not a April Fool's joke. Though it is also a funny occasion :)

We are glad to announce a new experimental contest on Codeforces, "Problem Parser Contest".

Have you ever made training sessions, using the previous contests' archives? Have you downloaded from the Internet contest archives to test for your solutions locally? If the answer is 'yes', then you couldn't help noticing that each organizer insists upon inventing his own way (format) to distribute problems. Yes, it is true!

I've prepared and organized a huge number of trainings based on the previous contests. It is often rather a monotonous task; one often has to write some scripts that rename the input/output files of the problem into the standard form. In archives they are called very differently: they can be river.in.1, river.out.1, river.in.2, river.out.2, etc. In another problem they can be tests/1.dat, tests/1.ans, tests/2.dat, tests/2.ans etc.

At some point I got fed up with it and wrote a script that told the patterns of paths to the test files, using some heuristics and the regularity I'd noticed. The script worked well, but it wasn't perfect.

Unusual tournament Manthan 2011 showed the weaknesses of the current rating formulas. After the end of contest the winner of the competition tourist has expected rating change equal to "-1":)

This has arisen due to reduction of the calculated changes in such a way, that total sum is equal to 0. That is, Gennady was in some pros before such changes, but as the sum of all the changes turned out to be positive, then all the ratings were a little reduced. As a quick dirty hack in the round I turned off this feature, leading to a one-time inflation. Even a small mistake: I noticed that participants in the second division get too many points after a good performance on "Div. 2 Only" rounds.

Last night I don’t want to sleep, so – I was working. I reread my old post "Contribution 2.0" (in Russian) and decided to implement some ideas. Taking into account the comments that everyone (no matter the color nor the position in the contribution) has the right to express their opinions in the form of pluses or minuses, I did not restrict users the opportunity to express opinion.

On the other hand, the rules of contribution changing depending on your votes have modified. Since we have already appeared bots, trolls and other vermin, I will not reveal details, but note that:

• Contribution may change after some time after the processing of your vote - it is normal.
• Not all your votes may be taken into account when calculating the contribution changing - this is also normal, we will defend ourselves from dirty tricks.
• contribution slightly scaled - the numbers got bigger. Maybe it's nicer?

Just want to say that the bots usage, following the principles of "blood feuds," etc. serve as a reason for your disqualification. Do not tempt fate.

Soon, the problems coordinator, the user RAD will leave official lists of both contributions and rated, as he is active member of the Codeforces team.

I wish you high contribution
MikeMirzayanov

We are delighted to inform you that Codeforces has collaborated with CodeFest'11, the annual international online coding festival of Computer Engineering Society, IT-BHU. CodeFest is a unique fest wherein concepts of mathematics, logic, artificial intelligence, algorithms, language syntax, etc. are required to be deployed in programming; these concepts manifest themselves in solving problems effectively and efficiently!

Codeforces is the programming partner of CodeFest'11. In fact, Manthan:, which is the algorithm intensive programming contest being organised by CodeFest will be hosted on Codeforces.

• Mathmania: A mathematical puzzle contest inspired by Project Euler. Main event to be held on 27th Feb, 2200 hours IST. Prizes worth 30,000 INR to be won.
• Manthan: An algorithm intensive programming contest. It is a Google sponsered event with Prizes worth 50,000 INR Event to be held on 13th March, 2200 hours IST. Registrations are on!
• Perplexed!: Constraints based C programming contest. Event to be held on 11th March, 1800 hours IST. Prizes worth 30,000 INR to be won.
• Virtual Combat: An educational game of robotic tank. Watch this out. Phase - 1 submission deadline - 7th March. Prizes worth 35,000 INR.

More than 2500 programmers from 59 countries have already registered for CodeFest'11. Come and be a part of the biggest coding extravaganza in Asia.

Feel free to contact us at [email protected] or reach us personally at:

• Mohit Bansal [email protected] (+91-930-505-3020)
• Saket Saurabh [email protected] (+91-945-282-5690)

Be free and Happy coding,
CodeFest and Codeforces team

1. What is Codeforces? What kind of a site/resource is it?
2. What should I do to participate in contests? Is preliminary registration required?
3. What are the rules of the contests?
4. What languages can I use to solve problems here? Are there any problem solving examples?
5. I would like to organize a Codeforces round with my problems. What should I do to achieve that? Are the authors entitled to get any reward?
6. What other rules are on Codeforces?
7. Which technical details are useful to know about the Codeforces' testing system?
8. What are the rating and the divisions?
9. What is contribution?
10. What blog posts are useful to take a look at?
11. I've noticed that the site contains the solutions of all the contestants and the previous contests' tests. How is it allowed to use them?

Questions and answers:

1. What is Codeforces? What kind of a site/resource is it?
   Codeforces is a project joining people interested in and taking part in programming contests. On one hand, Codeforces is a social network dedicated to programming and programming contests. On the other hand, it is a platform where contests are held regularly, the participant's skills are reflected by their rating and the former contests can be used to prepare. Codeforces constantly develops and we plan to improve the platform to give the participants the opportunity to organize their own contests, filling the project with learning content, developing Codeforces as a training and learning platform.
2. What should I do to participate in contests? Is preliminary registration required?
   Contests are regularly held on Codeforces. Participating in them is free and open to everybody. Every month we organize approximately six contests. To participate, you have to be registered on the site (if you have an OpenID or a Gmail account, then you won't even have to memorize the password) and register for the oncoming contest. Make sure that you are present in the list of the users, registered for the contest, before the registration ends. Usually, if you can't take part in the contest officially (e.g. if it's the contest for the second division and you are in the first one), then you can register for the contest to participate out of competition.
3. What are the rules of the contests?
   They are usually held according to the original Codeforces rules. If it is specially stated, then the International Collegiate Programming Contest rules ACM-ICPC or some other modifications can be used. In brief, on the contests held by Codeforces rules you write solutions to the problems that are tested during the contest on a very small number of tests. Those who have passed that set of solution tests, their authors can block (refuse to resend the solutions of this task in future even if they find a mistake). Such authors receive the opportunity to look through the sources of other contestants, look for mistakes there and suggest the tests on which these solutions do not work. Thus, you can hack somebody else's solution and earn points through it.
   After the contest all the solutions that have passed the pretests and haven't been hacked are tested on the final set of tests. The value of a task decreases during the contest (the faster you solve the problem, the more points you receive), unsuccessful hacks take off the points and the successful ones add them. Please take a look at the detailed version of the rules before participating in the contest.
4. What languages can I use to solve problems here? Are there any problem solving examples?
   Read the post "On the technical details on Codeforces".
5. I would like to organize a Codeforces round with my problems. What should I do to achieve that? Are the authors entitled to get any reward?
   Do you want to organize a round? That's great! We are very pleased to hear that. Please, read the post.
6. What other rules are on Codeforces?
   We don't have a clear Great Codeforces Code of Conduct. However, of course, here you should behave in accordance with traditional rules of behavior in public places and theme groups. The following rules are, of course, necessary to follow:
   • Don't create more than one account, if you have forgotten the password, use the password reminding system.
   • Do not use harsh, rude or misleading handle.
   • Do not use anybody photo except yours. It is uncultured and could confuse Codeforces users.
   • Don't be rude, don't insult other participants and administration, try to be polite and pleasant to communicate with.
   • When you take part in individual contests, don't talk about the problems with other contestants, don't use somebody else's code or insert it into your solutions. If the contest is a team one, discuss the tasks only with the teammates.
   • Don't try to destabilize the site's and the checking system's work. Your programs should only interact with the console (for the problems with the standard input and output) or the input and output data files.
   • Don't publish or spread your solutions and solution ideas during the contest.
7. Which technical details are useful to know about the Codeforces' testing system?
   The Codeforces system resembles classical Online Judges. The solutions are tested on the tests prepared beforehand (or the hacks suggested by other participants). As a result, you receive verdicts, the meaning of which is clear from the title. It should be specially noted that Codeforces does not have the "Presentation Error" verdict, this situation is regarded as "Wrong Answer". All the suspicious verdicts (testing error etc.) are not considered while evaluating the results. That is also true for the solutions that had fallen on test 1 (in the problems containing more than one test).
8. What are the rating and the divisions?
   When the contesters take part in Codeforces contest, they raise or lower their rating that reflects their ability to solve the tasks. The rating is a modification of Elo rating, several details can be read in a fuller form. According to the rating, the contestants are split into two divisions: the second one (the weaker one, amateurs) and the first one (the stronger one, pros). The contestants who don't take part in contests and those whose rating is below 1900 belong to the second division. The 1900+ rating means that you're part of the first division. Usually two types of contests are held on Codeforces: for the second division contestants (the first division contestants can take part there out of competition) and for both divisions. The first contest type contains simpler and learning-oriented tasks.
9. What is contribution?
   The votes for the posts and comments of a user change his/her contribution. The contribution is intended to show the usefulness of the community user. The contribution counting system is imperfect; in future the rules for its calculation will be changed.
10. What blog posts are useful to take a look at?
    • On programming languages and technical aspects
    • Codeforces rating
    • Codeforces contests' format
      
    • The November Revolution of Colors and Titles (Table of ranks)
      
    • Code preview
      
    • Problems' Materials Publishing
    • Unofficial Codeforces FAQ
11. I've noticed that the site contains the solutions of all the contestants and the previous contests' tests. How is it allowed to use them?
    In fact, we publish materials from the past contests and they can be used, for example, for individual lessons. Using the materials on other Online Judges, public contests, etc. is prohibited. Be sure to
    read the license before using the materials.
    In order to view someone else's solution, just click on his id on the page "Status". From the page with the list of tasks you can go to the list of correct decisions for the given task. At the bottom of the "Status" pages (and others with the lists of decisions) there is an option of sorting the solutions on different criteria.

 set line [gets stdin]<br> scan $line "%d" n<br> puts $n 

Hello!

This contest has been prepared by Artem Rakhov and other participants of Petrozavodsk Training Camp from Saratov State University. We stopped solving problems of the camp, missed the lecture by Vitaly Goldstein (do not be angry, Vitaly), but the round has been prepared and we are waiting for you - the participants.

Happy hacking,
MikeMirzayanov and team Codeforces

UPD:

• Problems
  
• Standings
• Winner: winmad

The year 2010 is over and of course, we are all ready not only to meet the Old Style New Year, which is a Russian national holiday. We are also ready to learn the names of those awarded by the Cormen Medal in 2010.

First, nomination "The Best 2010 Secondary School Contest Participant". And the winner is... (drum-roll) Gennady Korotkevich. He took part in 5 out of 6 WPS (Winter Programming School) Olympiads organized by the Codeforces and won each of them! He participated individually in the team contest, but nevertheless got ahead of all the contestants.

The Cormen Medal in "The Best 2010 Participant" nomination gets... Gennady Korotkevich! Over the previous year Gennady participated in 15 Codeforces rounds and won 5 of them. He also was among the five leaders in 9 rounds. The phenomenal success may be due to the fact that Gennady studies in the 10th grade. Bravo, tourist! Codeforces wishes Gennady to have new victories and achievements!

In the nomination "The Best 2010 Problemsetter" Natalia Bondarenko. This year Natalia spectacularly finished the career of an ACM ICPC participant (let me remind you that she got the final's golden and silver medals and won NEERC 2008). Natalia also studies in Saratov State University as a postgraduate student and is a huge help to Codeforces with organizing rounds, preparing interesting tasks. Her list of achievements includes preparing 5 contests, writing 19 tasks and helping to organize yet more. Thank you, natalia. We gladly give the victory in this nomination to you and wish you further fruitful work.

Thanks to all those who took interest in the project in the previous year. We are very glad that our contests find their participants who share their impressions on Codeforces pages.

Hello!

Tired to rest? Your help is needed.

During the period from December 31 until today I have made many changes in the Codeforces. The changes affected only the interior, so you will not notice any change in the user interface. Nevertheless, during this time was changed about 150 project files, database schema, some principles of data storage. In short, after all this, I'm not sure that everything works as expected (although, of course, I spent time on testing). For this reason I decided to organize Codeforces Testing Round #1, which will unrated and his only goal - a comprehensive testing of the project in conditions close to the real contest.

It will be three problems (possibly well-known, folklore), but I hope it will be pleasure to solve them.

Thanks for the help to Artem Rakhov, Maria Belova, and Max Ivanov

MikeMirzayanov.

UPD: Thanks to all. The round finished. We didn't notice any bugs. Write suggestions and opinions in the comments.

Happy New Year, Happy New Code!

Let the new year bring you not only the traditional happiness, luck and health, but little programmer's delight: complete solutions, advance of (semi)finals, rating increase and absolutely no need to debug because of the perfect code. Wish you beautiful ideas and implementations!

It is usual to make presents On New Year. I do not know, what can you prepare for Codeforces (problems? articles?), and we decided that on New Year's sometimes happens what does not happen on other days. In the New Year holidays (from 31 December to 3rd January) in the user interface (Profile -> Settings -> handle) you can change your handle to any other.

Have fun celebrating the holiday,
Codeforces team

P.S. Thanks to anatoly-rr for such a beautiful calendar.