In 6 hours 2nd Round of the VK Cup 2016 programming competition is going to happen! If you haven't registered for the round — don't worry! There is an extra registration!

The teams who has advanced from VK Cup 2016 Round 1 and VK Cup 2016 Wildcard Round 1 can participate in this round. The competition has regular Codeforces rules. Also in the same time with official round there is a regular rated Codeforces round played on the same problem set for participants from both div1/div2 divisions.

This round was prepared by AlexFetisov and winger. That is the first round which we have prepared as authors. We want to thank Gleb Evstropov (GlebsHP) for his help. Gleb is doing great job as Codeforces coordinator and I wanted to tell that one more time! Also we want to thank Kamil Debowski (Errichto), Mateusz Radecki (Radewoosh), Boris Minaev (qwerty787788), Pavel Kunyavskiy (PavelKunyavskiy) for their help in testing the problems and for great suggestions. Huge shout out for Mike Mirzayanov (MikeMirzayanov) for everything he has done for all of us!

To advance to Round 3 team should have a positive score and has score not less than the score of the 100th team in the final scoreboard. Also note that all teams advanced to Round 3 will get a special edition t-shirt of the competition. Also top-50 participants of the round 3 will get this t-shirt as well.

Good luck and have fun!

Update

Round has been finished. There were some problems during round but we hope that you enjoyed problems. Congratulations to the winners!

Official VK Round 2:

1. Who`s On First Base!: -XraY-, ershov.stanislav
2. Beer and lemon tea: sankear, Zlobober
3. MYCOPOBO3: V--o_o--V, LHiC
4. Never Lucky: subscriber, tourist
5. 33% less bad jokes: ifsmirnov, Arterm

Div1 results: Congratulations to anta who is the winner of this round div1 solving the hardest problem of the contest.

1. anta
2. jqdai0815
3. Petr
4. dotorya
5. ikatanic

Div2 results:

Congratulations to alexrcoleman who is the winner of this round div2 solving all problems less than in an hour!

1. alexrcoleman
2. nherceg
3. santjuan
4. mkisic
5. unused

Editorial http://codeforces.net/blog/entry/44538

Hello, Codeforces!

Note! The day of the contest here was wrong. Now it's fixed.

Educational Codeforces Round 12 will take place on 20 april 2016 at 18:00 MSK for the first and the second divisions.

The round will be unrated for all users and it will be held with extented ACM ICPC rules. You will have two hours to solve six problems. After that you will have one day to hack any solution you want. You will have access to copy any solution and test it locally.

If you have ideas for some problems or maybe already prepared problems that you can't use in rounds or official competitions, you can write to me.

It seems that it is harder to invent interesting simple problems (like A and B) than difficult ones. So don't be afraid to suggest interesting simple or very simple tasks (of course the difficult tasks are also needed). Please send only the problems for which you know the solution with clear statement (the story is not required). And also please add one or two samples to make the problem statement more clear.

The problemset was suggested by Codeforces users (it's time to lift this sentence inside the tegs). The problem А was suggested by user unprost. The problem B was suggested by Ayush Anand JeanValjean01. The problems C, D and E was suggested by Zi Song Yeoh zscoder. Sheikh Monir skmonir a long ago sent me a problem with difficulty C or D. I improved it by significant increase of the constraints (thanks to Mikhail Tikhomirov Endagorion who told how to solve such things). So the problem F was born.

Thanks a lot to them and all others who are sending the problems! The number of unused problems are increasing. If I didn't miss anything I already replied to all who sent me the problems at least 5-6 days ago. Please be patient if your problem was not used a long while.

The problems was prepared by me (Edvard Davtyan). Thanks to Maria Belova Delinur for checking the English statements. The problems was tested by users suggested them, respectively: unprost, Ayush Anand JeanValjean01, Zi Song Yeoh zscoder and Sheikh Monir skmonir. Thanks a lot to all of them!

I hope you will enjoy the problems! On my mind all the problems except of F is easier than usual, but F is harder.

Good luck and have fun!

Only 30 days left to the ACM ICPC World Finals!

It will be difficult to focus on the problems in such a beautiful place :-)

UPD1: The first part of the contest is finished. The open hacks will be opened in a few minutes.

UPD2: The editorial is ready for the problem F.

UPD3: The editorial is ready.

Hi!

We are pleased to announce that in 2016 Russian Code Cup goes international! Problems will be available in two languages: Russian and English, everyone is welcome to participate!

Championship rules and time table will be announced next week, meanwhile we would like to invite you to take part in Warm Up round on Saturday, April 23, 18-00 Moscow Time (UTC+3). Register at http://russiancodecup.ru and participate!

Good luck and see you at Russian Code Cup 2016!

Hello Codeforces!

The Final Round of CROC 2016 will be held today at 17:15 Moscow time, where the best 50 participants from the Elimination Round will be competing for the valueable prices, as well as for their personal entertainment.

Everyone else will be able to participate in Codeforces Round #347 tomorrow at 19:35 Moscow time that will feature an almost identical problem set. It is going to be a usual unrated round, separate for each division.

The problem set was prepared by Evgeny Vihrov (gen), the one and only coordinator of Codeforces Gleb Evstropov (GlebsHP) and me. I would also like to thank Mike Mirzayanov (MikeMirzayanov) and all of the Codeforces team for the incredible contest development system and Alexander Fetisov (AlexFetisov) for test-solving the problems.

During the Final Round the contest scoreboard will be linked here, but the problems themselves will be available only tomorrow.

We hope that you like our problems. Good luck to the finalists and to everyone else tomorrow!

UPD 1: The link to the current standings: http://codeforces.net/spectator/contest/662/standings

UPD 2: Codeforces Round #347 will be unrated.

UPD 3: Scoring:
Div 1: 500 — 1000 — 1500 — 2500 — 2500
Div 2: 500 — 1000 — 1500 — 2000 — 3000

UPD 4: Congratulations to the winners!

UPD 5: Editorial: http://codeforces.net/blog/entry/44408

Hello everyone,

Some days ago we had an annual programming contest in Samara, and yet again we copy it into Codeforces Gyms. The gym contest takes place on Sunday, 17 April, at 11:00 MSK. Site clist.by says that there are no intersections with something important that day.

Previously, it was a team contest, but this year we decided to make it personal. So we ask everyone to participate solo. This is how the results will be the most adequate.

And as usual,

The contest is over; the editorial is available here.

The language of this round is J.

The traditional A+B program (A and B are written in one line and separated with space) looks as follows:

print =: 1!:2&2
read =: 1!:1[3

in =. (read-.LF)-.CR
print +/ ". in

exit ''

The main source of information about the language is http://code.jsoftware.com/wiki/Main_Page. Version used is J804.

Hello, Codeforces!

Educational Codeforces Round 11 will take place on 8 april 2016 at 18:00 MSK for the first and the second divisions.

<The changes in the last paragraph>

The round will be unrated for all users and it will be held with extented ACM ICPC rules. You will have two hours to solve six problems. After that you will have one day to hack any solution you want. You will have access to copy any solution and test it locally.

If you have ideas for some problems or maybe already prepared problems that you can't use in rounds or official competitions, you can write to me.

It seems that it is harder to invent interesting simple problems (like A and B) than difficult ones. So don't be afraid to suggest interesting simple or very simple tasks (of course the difficult tasks are also needed). Please send only the problems for which you know the solution with clear statement (the story is not required). And also please add one or two samples to make the problem statement more clear.

</The changes in the last paragraph>

The problemset was suggested by Codeforces users. The problem А was suggested by Ali Ibrahim C137. The problem B was suggested by Srikanth Bhat bharsi. Mohammad Amin Raeisi Smaug sent the problem C. The problem D was suggested by Sadegh Mahdavi smahdavi4 a long ago. The problem E is the last (the fourth) problem suggested by Lewin Gan Lewin. The problem F is the last (if I'm not wrong the third) problem suggested by Kamil Debowski Errichto.

Thanks a lot to them and all others who are sending the problems! At this point I've checked all the problems sent to me (except maybe for the problems sent in last week). I hope that I've replied to all of you. I have a list with all problems sent to me, so be sure I don't forget about your problem.

The problems F was prepared by Kamil Debowski Errichto. Other problems was prepared by me (Edvard Davtyan). Thanks to Maria Belova Delinur for checking the English statements. The problems was tested by users suggested them, respectively: Ali Ibrahim C137, Srikanth Bhat bharsi, Mohammad Amin Raeisi Smaug, Sadegh Mahdavi smahdavi4, Lewin Gan Lewin and Kamil Debowski Errichto. Thanks a lot to all of them!

I hope you will enjoy the problems! I'm considering the possibility to copy the problem E with larger constraints as problem G. What do you think about that?

Good luck and have fun!

P.S.: The bus from the problem B.

UPD: The contest is finished. Congrats to tribute_to_Ukraine_2022 and uwi with the win and halyavin with 93 hacks! The editorial is ready.

Hello!

Last weeks I was concerned (and probably, you too) about abnormal rating growth of our leaders. Of course, first of all I'm talking about tourist, his rating is just rushed into the sky.

There were even comments from a series of "I told, and it happened"

After the first round of VK Cup 2016, I carefully investigated the reasons of such growth and found a simple and trivial bug in the rating formulas. It's funny that even after being published, nobody found it. Look at this function:

    private double getSeed(List<Contestant> contestants, Contestant contestant, int rating) {
        Contestant extraContestant = new Contestant(null, 0, 0, rating);
        double result = 1;
        for (Contestant other : contestants) {
            result += getEloWinProbability(other, extraContestant);
        }
        return result;
    }

Remind, this feature is to calculate the expected participant place, if its rating suddenly became equal to rating. Of course, it should not take account of the participant itself (to whom we currently assign new hypothetical rating). The correct code must be:

        for (Contestant other : contestants) {
            if (other != contestant) {
                result += getEloWinProbability(other, extraContestant);
            }
        }

This bug led to the fact that taking the first place tourist actually won a very serious opponent. Himself. This led to a significant increase in its rating, even if the first was quite expected.

The good news is that this bug has a statistically significant effect only in the very rare cases when the winner (or close to the winner) had a very high rating (yes, contrary to "anti-heroes" is also true). If we take an arbitrary round and recalculate the rating formulas corrected, almost all participants will receive exactly (or very close) rating.

After consulting with tourist and Petr, I came to the following plan of action:

• Today we had chronologically recalculated all ratings from the revolution of 2015,
• If difference between the change according to the corrected rating formulas and the historical change (according to formulas with a bug) is no more than 3, the historic change continued to be used,
• If difference between the change according to the corrected rating formulas and the historical change (according to formulas with a bug) is more than 3, the change is replaced with the correct rating in history.

It turned out that this bug did not affect practically all users. Bug touched only the top. I apologize to those who had descended from heaven to earth — but it was impossible to leave it as is. I wish leaders to get those ratings that were before rating fix.

Mike.

The contest is over; I hope you've enjoyed it :-) Editorial is here. See you next year!

The fourth April Fools Day Contest will take place on Friday April 1st. This is a joke competition in which solving the problem is often easier than figuring out what the actual task is. Thanks to kit1980 and Codeforces team for their help in preparing problems.

In this round you'll be given 7 weird problems and 2 hours to solve them. The contest will use ACM ICPC rules (no hacks, the standings are decided by the number of solved problems and penalty time earned on them), and it will be unrated. You can submit solutions in any language allowed by Codeforces — well, unless the problem says otherwise ;-) To get an idea of what the contest will look like, you can check out the contests of the past years: 2012, 2013, 2014.

Be warned, to enjoy competing in this round you'll need a sense of humor compatible with mine! Good luck, and have fun!