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.

Hi everybody,

I am happy to tell you that Codeforces will soon free you from reading those endless "Please, give me test #??" in the comments. Now you can click on a link with the submission-id on the status page and it will show not only the original solution texts, but also the tests on which testing was performed (the judging system report). It will not contain the full inputs/outputs/answers if they are too large – you will have to wait for now and enjoy their prefixes. As soon as we understand that there're no bugs in the implementation, we will open all the previous contests to view the tests.

However, that is only the first step; later you will be able to download contest problem archives that will contain full texts (or their generators) as well as the authors' solutions and other materials.

Of course we do not want Codeforces problems to be copied from one "online judge" to another one. You’ve got to understand that Codeforces puts huge effort into preparing tasks and organizing contests. That's why we limit the material usage to make using our problems on websites or in other automatic testing systems impossible. On the other hand, we will be glad if the contest materials will be useful for individual training or for some assemblies, training sessions etc. with the onsite participants. Read the license text carefully as we want some Codeforces data to be published near (or inside) the problem tasks.

At the moment it is version 0.1 of the license, we will perhaps change a lot in it, but I guess the idea will undergo no changes.

Codeforces materials usage license (v. 0.1)

1. You may publish the texts of Codeforces problems in any open sources, but you must preserve a direct link to the site http://codeforces.ru (http://codeforces.com for English statements) and indicate Codeforces as the problem source. These data should be located in the close proximity to the statement, be easily noticeable and readable. It is forbidden to publish Codeforces problems in open sources supporting automatic testing (such as online judges or similar resources).
2. You must not use tests and other materials of the problems (generators, checkers, validators) to publish tasks in open sources supporting automatic testing (such as online judges or similar resources) even provided the original problem conditions has been changed.
3. You may use the provided material: tests, generators, checkers, validators and others for individual training or to organize short-term teaching and training activities with the participants present on-site. Task conditions should contain the data from paragraph 1 of this license. The secondary (subsequent) distribution of the tasks should contain this license and comply fully with the license’s rules.

Good evening… (good?)

There were difficulties with the round, for which I apologize. During the contest I was able to partially locate the troubles, I hope we’ll fix them soon. Apparently, there were two troubles: 1) site didn’t respond, 2) for hacking attempts occurred inappropriate message that hacking is already taken. We will investigate it. Thank you for your patience.

So we decided to do it unrated.

I added work to the next round, the more so because I'm going to be its author:)

MikeMirzayanov

One of the most important all-Russia (it's even greater! It's all-region) sport programming contests has approached us. Of course, I mean the ACM-ICPC NEERC 2010/11 World Programming Championship semi-finals. The names of those who will go to the jewel of Egypt, Sharm el-Sheikh will be known in less than a week. Undoubtedly, for some people this contest will become "the flop of 2010". I hope not to become one of them.

As you already know, the Saratov university delegation is going to get on the train tomorrow with some special extra luggage - we are taking about 300 T-shirts with us. Today we spent half a day sorting them into boxes, signing and counting them again. I even managed do divide 68 by 2 incorrectly that nearely led to the shortage of T-shirts :) However all the hardships are over, the presents are packed and we are ready to go. Let me remind you that all the official team members will receive Codeforces T-shirts (unfortunately, the reservists, coaches and the team leaders do not count).

The previous year in St. Petersburg was marked by our hasty visits to all the nearby pharmacies, it was marked by the fact that on the eve of the contest every team had a member with the fever of 38 degrees celsius, it was marked by the fact that one can never have enough paracetamol before the contest. We don't wish anybody to share this wonderful experience, so dress warm, take care of yourselves and of your teams' health!

Have a good journey and see you in St. Petersburg,
МikeMirzayanov

This year the ACM-ICPC regional contest of our region celebrates its 15th anniversary! We heartily congratulate the organizers, members of the jury and the technical committee. We would like to give our special congratulations to the Competition Director Vladimir G. Parfenov. Thank you very much for your work not only in organizing this competition, but also in developing the programming community throughout our region. Much has been done over 15 years. All the achievements of our region teams on the ACM-ICPC finals are also due to you. I have to say that NEERC is organized on the top level and has highly interesting sets of problems which all the participants find very exciting. We wish you new successes and new achievements!

This year the Saratov university delegation does not arrive in St. Petersburg empty-handed. We've decided to give presents to all participants. Each St. Petersburg NEERC participant will receive a Codeforces T-shirt. For us, delivering so many T-shirts will help to clarify who among our team members is the strongest programmer:)

The Codeforces team and Saratov University.

School Team Contest #3 (Winter Computer School 2010/11) will be held on November 13 at 11:00 (UTC). This is the final team event of the series, and we will be glad to see both permanent participants and new teams.

The contest has been prepared by me, Artem Rakhov, Nikolay Kuznetsov and Ivan Fefer. All of us will soon go the ACM-ICPC regionals in St. Petersburg, and now the guys have to combine the preparations for the semifinals with writing problems for you. Special thanks for the translation of problems to Maria Belova.

Everyone can take part in it out of the competition (informal). Make up your mind :)

We decided to sum up contest results using the ITMO rating system, according to which team gets a score from 0 to 200 per contest. It will be used two best participations out of thee possible. I will not reveal secrets, saying that Gennady Korotkevich showed great results and secured the victory ahead of schedule!

Good luck in the upcoming competition, MikeMirzayanov and stern, but fair jury.

UPD. Statements in PDF: russian version and english version. The statements will be available when the contest starts. 

Good day!

We are pleased to inform you that on the night of December 31, 2010 on January 1, 2011 will be not only fireworks, champagne, gifts and presents, but we will summarize the Codeforces year. Great news that Thomas Cormen gave permission to use his name in Codeforces annual award. So, we are pleased to announce the annual Codeforces award — the Cormen Medal!

It will be three awards this year:

• the best Codeforces participant
• the best secondary school Codeforces participant (based on Winter Programming School Olympiads for Schoolchildred results)
• the best problemsetter

The first of the medals will be awarded to participant who will have the highest rating at the end of the year. The second medal will be presented to the best participant of Winter Programming School Olympiads for Schoolchildred 2010/2011. Finally, the third prize will get the most active and successful author of problems for Codeforces competitions (Codeforces team members not considered in the tabulation of results).

Wish you high ratings,,
MikeMirzayanov and Codeforces team

Hello.

Now you can create teams and invite other users Codeforces. I remind you that the first distance Winter Computer School Olympiad will be for teams, and for official participation in it only teams will be allowed (all team members should be registered to the serie).

P.S. It is only the first version of the teams interface. We are working on extending it. Suggestions are welcome.