Hello, Codeforces!

Actually, I didn't understand how, but I've prepated a round for the second division. I think it is subconscious wanted to soften the hard memories of the round of 276.

I hope that very much that the round will be held without any technical issues, as was the 277th.

I express my sincere gratitude to all involved in the preparation — Maxim Zlobober Akhmedov, Maria Delinur Belova, Polygon System, servers and James Gosling for Java. Also I am immensely grateful to the driver, who did not knock down me by car, although I, thinking about the problems, was crossing road on red.

It will be rated round, 6 problems and non-typical progressive scores: min(500 + i*500, 2500).

Wish you to see many Accepteds.

UPD. The Round moved 5 minutes forward. I want very much so that no one be late! Sorry.

UPD 2. Rating has been updated. But if we will find any cheaters, they will be punished and we will recalculate ratings. Thank you for participation!

Welcome to 2014-2015 CT S02E09: Codeforces Trainings Season 2 Episode 9 - 2006-2007 ACM-ICPC Northeastern European Regional Contest (NEERC 06). The training duration is 5 hours. It is opened for teams as well as for individual participants. After the end you may use the practice mode to complete problem solving. Also it will be availible as a virtual contest for whose of you who can't take part today. Please, do not cheat. Only fair play!

It is possible that the problems will be too easy for some participants, it is possible that we will add some problems.

Good luck!

Hello,

I'm really starting to think that plans to host ACM-ICPC World Finals bring misfortune. ACM-ICPC World Finals 2011 initially were planned to be in Egypt, but revolution has changed plans.

What do you think about chance to drop World Finals in Morocco because of Ebola Virus?

Welcome to 2014 Benelux Algorithm Programming Contest (BAPC 14), 2014-2015 CT S02E08: Codeforces Trainings Season 2 Episode 8. The training duration is 5 hours. It is opened for teams as well as for individual participants. After the end you may use the practice mode to complete problem solving. Also it will be availible as a virtual contest for whose of you who can't take part today. Please, do not cheat. Only fair play!

It is possible that the problems will be too easy for some participants, it is possible that we will add some problems.

Good luck!

Hello Codeforces.

Just a reminder about something most of you know. Polygon is a service to prepares programming problems and contests. It is usually used to prepare problems for olympiads/programming contests but it is also often used to prepare educational content. It is located at https://polygon.codeforces.com/ and is open to everybody.

I have recently found out that it's been more than five years since the creation of Polygon. It's high time to summarize the experience we've accumulated.

I first publicly spoke about Polygon in a narrow circle of Russian teams' coaches on ACM-ICPC World Finals in 2009. I cannot say that everybody was enthusiastic about the innovation. Some people were openly skeptical about such system's viability and relevance. After 5 years on the ACM-ICPC finals in Ekaterinburg Oleg Hhristenko (snarknews) said that in his opinion, the creation of Polygon is more of my achievement than Codeforces. Of course I was surprised but I said so to Polygon :-)

When I started working on Polygon (in the autumn of 2008), I understood very well how much this system was needed. By that time I had already been an experienced writer of many problems. The infinite number on highs-school Saratov Olympiads and ACM-ICPC Subregionals. My problems were on the All-Russian Team School Student Olympiad in Programming, the Russian National Olympiad in Informatics (for High-School Children), the ACM-ICPC NEERC Regionals, the TopCoder Open Finals and many other places. In most cases the problems were developed in some VCS (e.g. SVN) everything was structured by some half-formal unspoken naming rules.

I am going to give you some thesis statements on why it is good to use Polygon and bad not to.

1. Polygon protects from errors

Polygon is full of automatization, self-checks and verifications. Some examples:

• it is hard to make a misprint in a sample input/output and forget to actualize it after the tests are changed as it is inserted automatically and the answer is generated by Polygon using the model solution;
• it is hard to leave a non-compiling solution in the archive (even experienced teams like ITMO regularly leave the solutions on Java in the archive where the name of the class doesn't match the name of the file);
• it is hard to forget to make the first test the statement test, Polygon would warn you about it;
• it is hard to write a generator that is initialized from time and thus it prints different tests on consecutive invocations (i.e. not stable), Polygon will run the generator a couple of times with an interval of one second and check if the tests match.

2. The archives (packages) of the Polygon problems are uniform and machine-readable

It's striking that the olympiad community hasn't standardized a method to distribute problems. The problems from Polygon have a uniform and logical way of organizing files and are machine-readable. File problem.xml contains not only the basic meta information like memory limits, but it also has everything you are going to need for the continuing work at a problem. Here are some examples:

• for TL it shows the type of the processor for which the TL was chosen;
• it clearly shows the IO method and file names (if used);
• it supports multiple native languages in for problem title/statements;
• It has an exact way to generate each of the generated tests;
• solution tags (for example, a deliberately slow solution can be tagged as time-limit-exceeded);
• precise paths to tests and other resources.

I don't know any other machine-readable descriptor formats in use that are so complete.

3. Polygon stores files for long-term and keeps them available

I can open the problems of the ACM-ICPC Saratov Subregional 2009 and edit a test. Everybody who has access to the problem can do the same. All the changes will be visible to all the coauthors, they will be notified via email and the automatic testing systems can get the changes when new problem package is built. I can run the solutions as I edit, all tests will be validated, all the solutions will be ran. But what happens if you don't use Polygon? For the time of contest developement a local version control system is used, it is usually shut down later and only the archive remains: we lose editing history, developing access, automated invocations and other things.

4. Polygon decreases the barrier to entry the problem preparation process

The Codeforces problem writers are the contest participants of different level of training, cultural and professional background, for many of them it's their first experience of preparing problems. They almost always easily understand what to do and how to do it, they can add problems. If you don't use Polygon, then the process is usually regulated by a system of unspoken rules (like, add suffix _slow, or _tl to the title of the slow solution), an unexperienced participant gets lost and cannot understand what to begin with and how to work. Besides, it usually requires some knowledge of basic principles of working with svn and command line and work differs for the Windows and Linux users.

5. Polygon helps you manage access

In Polygon you can give access to the problem to any registered user. If you work without Polygon, it is usually done by the version control system server administrator and you don't even have the list of your co-authors. You cannot manage access on your own, the access is usually managed at the contest level, not problem level.

6. Polygon has issue-tracking

When there is more than one developer working simultaneously, they absolutely need to be able to add and manage issues. Besides, issue-tracking is useful in individual work as well. It structures work, helps you not to lose track of the details. Whenever I've worked outside Polygon, the best task management I saw was a special file with a numbered list of tasks.

7. Polygon is easily integrated with online judges

Polygon offers machine-readable problem packages with prepared tests (for both Windows and Linux) or enables to generate them at package deploying. Polygon has a simple HTTP POST-based API to access data. While POST-queries are formed, you need to set the user's login and password parameters (and optionally, revision).

• https://polygon.codeforces.com/p/username/problemname — download the last package (unless the revision is defined);
• https://polygon.codeforces.com/p/username/problemname/problem.xml — download the descriptor from the last package (unless the revision is defined). For example, in order to see that the revision has changed and we need to re-download the package;
• https://polygon.codeforces.com/c/contestid/contest.xml — download the descriptor of the contest
• https://polygon.codeforces.com/c/contestid/english/statements.pdf — download the PDF with statements in English, all the statements are listed in the contest's descriptor.

Say no to manually copying problems into the judging system, let the system upload the problem for you and deploy or update it.

8. Without Polygon the developer needs to have some special soft on his PC

For example, a developer with Windows may need bash to run doall.sh that generates all the tests. Or a Java programmer will have to install C++ to compile the checker. Use LaTeX to compile the statement. Polygon sets you free from all of that, lots of work is done on the side of the server.

9. Polygon is safer than most alternative ways of shared development

Polygon uses HTTPS, you can attach browser session to IP-address (it is optional, of course), session is attached to User-Agent, we use CSRF-tokens everywhere.

10. Polygon classifies and indexes problems

You will never get lost in problems. I have about 500 problems in Polygon made by me only, but due to tags, filters, search and sorting into contests I never get lost in them and can quickly find what I need.

The End

Those are only the first reasons that came to my mind at 2 AM. I am sure that we can come up with some more of them after some thorough thinking.

The conclusion is simple. Every time you make a problem not in Polygon, god kills a kitten.

Welcome to 2014-2015 CT S02E07: Codeforces Trainings Season 2 Episode 7. The training duration is 4.30 hours. It is opened for teams as well as for individual participants. After the end you may use the practice mode to complete problem solving. Also it will be availible as a virtual contest for whose of you who can't take part today. Please, do not cheat. Only fair play!

It is possible that the problems will be too easy for some participants, it is possible that we will add some problems.

Good luck!

The contest has been moved to the Gym.

Today it will be ACM-ICPC, NEEERC, Southern Subregional Contest! As a head of judges I wish good luck to all the participants!

• standings link: http://contest.sgu.ru/monitor/?id=1

On Saturday (October, 25), 07:00 (UTC) it will be a internet-mirror: [contest:481]. Interesting problems are waiting for you. Judges tried to prepare problems of wide difficulty range: for newcomers and for expirienced teams.

For sure, it will be unrated contest. We recommend you to take part in teams. I think, the contest will be moved to Gym later.

Good luck!

MikeMirzayanov, head of judges.

P.S. Saratov Views:

Because of the Codeforces Round 273 (Div. 2) the training contest 2014-2015 CT S02E06: Codeforces Trainings Season 2 Episode 6 - 2007 Benelux Algorithm Programming Contest (BAPC 2007) will be moved one day forward.

Welcome to 2014-2015 CT S02E05: Codeforces Trainings Season 2 Episode 5 - 2009-2010 ACM-ICPC, NEERC, Southern Subregional Contest. The training duration is 4.30 hours. It is opened for teams as well as for individual participants. After the end you may use the practice mode to complete problem solving. Also it will be availible as a virtual contest for whose of you who can't take part today. Please, do not cheat. Only fair play!

It is possible that the problems will be too easy for some participants, it is possible that we will add some problems.

Good luck!

Welcome to 2014-2015 CT S02E04: Codeforces Trainings Season 2 Episode 4 (US College Rockethon 2014 + COCI 2008-5 + GCJ Finals 2008 C). The training duration is 4.30 hours. It is opened for teams as well as for individual participants. After the end you may use the practice mode to complete problem solving. Also it will be availible as a virtual contest for whose of you who can't take part today. Please, do not cheat. Only fair play!

It is possible that the problems will be too easy for some participants, it is possible that we will add some problems.

Good luck!

Welcome to 2014-2015 CT S02E03: Codeforces Trainings Season 2 Episode 3 (NCPC 2008 + USACO DEC07 + GCJ 2008 Qual). The training duration is 4.30 hours. It is opened for teams as well as for individual participants. After the end you may use the practice mode to complete problem solving. Also it will be availible as a virtual contest for whose of you who can't take part today. Please, do not cheat. Only fair play!

It is possible that the problems will be too easy for some participants, it is possible that we will add some problems.

Good luck!

Welcome to 2014-2015 CT S02E02: Codeforces Trainings Season 2 Episode 2 (CTU Open 2011 + misc). The training duration is 4.30 hours. It is opened for teams as well as for individual participants. After the end you may use the practice mode to complete problem solving. Also it will be availible as a virtual contest for whose of you who can't take part today. Please, do not cheat. Only fair play!

It is possible that the problems will be too easy for some participants, it is possible that we will add some problems.

Good luck!

Welcome to 2014-2015 CT S02E01: Codeforces Trainings Season 2 Episode 1 (NEERC 99 + misc). The training duration is 4.30 hours. It is opened for teams as well as for individual participants. After the end you may use the practice mode to complete problem solving. Also it will be availible as a virtual contest for whose of you who can't take part today. Please, do not cheat. Only fair play!

It is possible that the problems will be too easy for some participants, it is possible that we will add some problems.

Good luck!

The new season of a collegiate team championship ACM-ICPC is about to start. For example, the registration for the Southern (Saratov) Subregional Contest is already running. I am sure that many participants of the Codeforces rounds will take part in ACM-ICPC this year.

We are launching a series of weekly practice trainings on Codeforces. Naturally, they will be held within Codeforces::Gym. Feel free to participate!

The practice starts on Thursdays at about 12:10 PM (UTC), which is 16:10 Moscow Time. Expected duration is 4-5 hours. We are going to practice using the problems of different contests of the past years. All you need is common sense and observing these simple rules:

• We will not publish the problem source before the practice starts. We want you to solve the problems on your own in a fair competition. If you use somebody else’s code or cheat in any other way, you will be disqualified. If you don’t want to solve on your own, that’s fine, you don’t have to. But spoiling the practice for others is unacceptable.
• Do not discuss the problems till the practice ends.
• We will rarely answer the questions about problems. If you’ve found some obvious bug, please let me know. We will fix the bug and send everybody the note about the fix.
• If you have a coach account (and you do not participate in the practice), we will be grateful for your help.
• Please register for the practice with the people from your team who actually participates in it.
• From time to time, I am going to ask some of the jury of the past contests or coaches from other higher educational institutions to help with preparing or share materials — your understanding and help will be greatly appreciated!
• if you solved the contest problems before just switch to another training or inform us via problem questions form, we will move you to out-of-competition role.

The first contest 2014-2015 CT S02E01: Codeforces Trainings Season 2 Episode 1 (NEERC 99 + misc) takes place on September, 11, at about 12:10 PM (UTC).

I've got challenge from agul and caustique. Challenge accepted!

Challenge thrown to TopCoder employees: rng_58 and ivan.metelsky.

Dear friends, you might have noticed that the VK logo has disappeared from the Codeforces pages and all of a sudden the place got sort of empty and a little bit gloomy. By the way, it happened not because the VK company lost the interest towards Codeforces and programming contests. Not at all. On the countrary, we have great plans to carry out together! We are infinitely grateful to the VK company for the 4 years of help and support.

We are glad to inform you that now a wonderful company called Telegram is going to help Codeforces. The company is founded by Pavel Durov and it united many brilliant developers with a rich olympic past. It is pleasant and important for us to see Codeforces helped by the people who understand programming contests, love them and value the skills of the community members. The Codeforces team is happy to get the opportunity to continue working and promises to continue dazzling you with contests, innovations and improvements.

Mike Mirzayanov and the Codeforces team

Oh yes! Only a few days left before the Championship finals! The teams have already gathered in Yekaterinburg, most of them have registered and are watching a game between Russia and Belgium.

I want to start from some history and remember that the tradition to publish travel notes about Saratov State University's trips to finals started back in 2005. The regular pattern is that almost every year when we made notes, our team won a medal. I won't try my luck, so here are some of the first impressions of this year.

We've introduced API and now we want to test the system before Round 251.

I invite you to take part in Testing Round 10. It starts on usual time, June 3rd. It will be unofficial unrated round.

I tried to pick up the problem to make the round interesting for many of you. Pretests are unusually weak to trigger more hack.

If you see any unexpected behavior or bugs, please inform us via comments.\

Thanks.

After a short delay (but Oscar is given in spring too, huh?) we are pleased to announce the Cormen Medal laureates for 2013. This year, we’ve decided to slightly upgrade nominations again, so this year's Cormen Medal is given in two nominations:

• Best Progress
• Best Problemsetter

Best Progress

The Cormen Medal laureate in this nomination is Scott Wu (scott_wu, USA). Note the sharp upward dive his rating line takes. His achievements in 2013 are not limited by the spectacular dive into the best 10 participants on Codeforces: he got the 5-th place on IOI, won the 2013 season of the USACO contests, got target on TopCoder! We congratulate Scott and wish him many more achievements!

Best Problemsetter

We didn't have to search far and wide for the winner in this nomination. Naturally, the Cormen Medal goes to the most productive and loved by many author of 2013, Sergey Nagin (Sereja, Ukraine). Sergey prepared and conducted 7 rounds (all of them Div1+Div2) on Codeforces. Sergey's problems gained popularity among the Codeforces coders and we will be happy to see his contests again in the future! Sergey has already been awarded by a Medal and an Award Plate in February in Kharkov training camp.

Looks like it's becoming a good tradition. The laureates will be sent a book by Thomas Cormen (Introduction to Algorithms or Algorithms Unlocked), signed by the author.

History

Let us remind you that the Cormen medals have been awarded for the fourth year. tourist (Gennady Korotkievich) became the best participant three years in a row (in all the years when this nomination existed). Alex_KPR (Alexander Kouprin) became the best blogger in both years when this nomination existed. My favourite nomination "Best Problemsetter" was awarded to: natalia (Natalia Bondarenko) in 2010, Ripatti (Artyom Ripatti) in 2011, and witua (Vitaly Gerasimov) in 2012. Besides, in 2012 there was a medal for "Codeforces Spirit of Community 2012", the laureate was Nickolas (Mariia Mykhailova).

https://get.google.com/albumarchive/pwa/114907919772955385569/2014CoderStrike?authuser=0&feat=directlink

Hello,

Once again I'd like to host unusual unrated round. With your help we want to test untypical scoring system and unusual problems.

The contest will start today on 18:30 (UTC). We will add large and bright special link to enter into the contest area. The contest duration is 90 minutes. But it will be easy problems and hope that many participants will solve all of them before the end.

Go to the Contest →

It will be two types of problems: logic puzzles and programming tasks.

A logic puzzle is a task that is designed to be solved by hand (but it is not forbidden to write some helper code to do this). Each logic puzzle consists of one or more tests. Each question can be answered separately from the others. There is a "Submit" button below each test. To answer the question, press "Submit" button. Your answer will be acknowledged and checked after the end of the contest. You can change your answer any number of times – only the last attempt is checked. For the correct answer to the question you will get the number of points which is indicated in the question description.

A programming task is which you usually see in Codeforces rounds. Solutions can be submitted at any time during the contest. A solution is evaluated on a fixed set of tests right after it is submitted. For each passed test, a contestant will get a fixed amount of points. The sum of points for all passed tests is the total points received by the solution. The contestant can submit a solution several times. The contestant will receive points for only one solution per problem. The solution with the maximum amount of points will be chosen.

If a pair of contestants have the same number of points gained in contest time, they will be ordered according to the time (in seconds) of the last submission which gave them an improved positive score.

For each programming task, we will consider only the best submission (or if there are many submissions which are equally correct, the earliest). For logic puzzles, we will judge last submission per test.

In other words, it's always safe to submit a new answer for programming tasks (you can only improve your situation). It is not always safe to do that for logic puzzles – consider carefully whether or not you want to do that (you may replace correct answer with incorrect).

Many thanks to all of you who will help. Waiting for your feedback in comments.