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!

Hello, Codeforces!

I'd like to invite you to Codeforces Round #346 (Div. 2). It'll be held on Wednesday, March 30 at 16:05 (UTC) and as usual Div. 1 participants can join out of competition. Note that round starts in the unusual time! Also, the round itself will be unusual: there will be 7 problems for two and a half hours.

Me (IlyaLos), Edvard Davtyan (Edvard), Dan Sagunov (danilka.pro), Roman Glazov (Roms) and Vladimir Petrov (vovuh) prepared the tasks for this Round. Great thanks to Gleb Evstropov (GlebsHP) for helping us preparing the contest, to Maria Belova (Delinur) for translating the statements into English and to Mike Mirzayanov (MikeMirzayanov) for the great Polygon platform, for helping on preparing the contest and for ideas for some tasks.

The scoring distribution will be announced later. Good luck everyone!

UPD Score distribution: 500-1000-1000-1250-1500-2000-2500

UPD2 Competition completed! Thank you all! Editorial

Hello everybody!

I'm glad to announce that Round 1 of VK Cup 2016 will take place this Monday, and me (Radewoosh) and Kamil Dębowski (Errichto) are the problemsetters!

There will be an official round for teams from VK cup, but if you are not eligible to participate in it, then you can compete (alone, not in team) in one of two additional editions (one for div.1 and one for div.2), so everybody is invited to take part in the competition! Just register in your category here. All three rounds will be rated. Div.1 and Div.2 editions will look like normal CF round, but will have common problems with official edition.

If you can't register before the round, then you will be able to do it during the contest (but not for the entire duration, you can cheсk it here). Let's thank Mike for this great feature!

We want to thank GlebsHP for help in preparing the problems and MikeMirzayanov because without him we wouldn't have such a great platform as Codeforces, where we all can train and develop our passion.

You will again help Limak, your favorite bear. This time it may be harder, because evil Radewoosh will try to disturb him.

We wish you good luck and great fun! Can't wait to see you during the contest! :D

UPD Scoring will be:

For VK: 500 — 750 — 1000 — 1500 — 2000 — 3000

For Div.2: 500 — 1000 — 1500 — 2000 — 2500

For Div.1: 500 — 1000 — 1500 — 2000 — 3000

UPD Editorial is ready.

UPD Congratulations to the winners!

In official VK:

1.Never Lucky: subscriber and tourist
2.SobolevTeam: Seyaua and sdya
3.LNU Penguins: witua and RomaWhite
4.Dandelion: Um_nik and sivukhin
5.uıɟɟnɯ ɐuɐuɐq ǝɥʇ ɟo uɹnʇǝɹ╰(º o º╰): enot110 and romanandreev

In Div.1

1.dotorya
2.kcm1700
3.JoeyWheeler
4.KrK
5.Swistakk

And in Div.2

1.osmanorhan
2.nhho
3.fudail225
5.agaga
4.alanM

Also let's thank qwerty787788 and AlexFetisov for testing problems, without them it would be much harder to prepare contest, so give them an applause!

Hello, Codeforces!

[Note the unusual (for the educational round) start time!]

Educational Codeforces Round 10 will take place after a long delay on 25 march 2016 at 16:00 MSK for the first and the second divisions. The long delay is caused by the high frequency of contests and championships on Codeforces. You can read about educational rounds here and here.

<The same as before>

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.

</The same as before>

From now traditionally the problemset was totally suggested by Codeforces users. The problem А for the third time was suggested by user unprost (no hope for the short problem statement :-)). The problem B was suggested by user Smaug. The problem C is another problem from the set sent by Bayram Berdiyev bayram, Allanur Shiriyev Allanur and Bekmyrat Atayev Bekmyrat.A. Alexey Dergunov dalex suggested the problems D and E. The problem F was suggested by Lewin Gan Lewin.

Thanks a lot to them and all others who are sending the problems or just ideas of the problems!

The problems D and E was prepared by Alexey Dergunov dalex. Other problems was prepared by me (Edvard Davtyan). I want to note the generator for the problem E its difficulty comparable with the solution to the problem F. Thanks to Maria Belova Delinur for checking the English statements. The problems was tested by users: unprost, Smaug, Aleksa Plavsic allllekssssa, Alexey Dergunov dalex and Lewin Gan Lewin. Thanks a lot to all of them!

I hope you will enjoy the problems!

P.S.: I really like the problem F and hope to see Accepted-s for it.

Good luck and have fun!

UPD: The contest is finished. The editorial is ready.

Hello.

Now it became much unlikely to skip a round due to carelessness during the registration. In the coming rounds (in experimental mode) we will add new feature extra registration (it is good terminology? additional registration?).

This means that after the period of regular registration (as usual, 5 minutes before the start of a round it will be closed) there will be another additional period of registration. It will start in 10 minutes after the start of the round with duration of 20 minutes, that is, it will close after 30 minutes from the start of the round.

The role of the 10 minutes is twofold:

• on the one hand it is quite possible to solve the easiest problem in the first 10 minutes — so it makes a motivation to register on time,
• on the other hand the system is loaded much during the first minutes of a round, as a registration invalidates some internal caches it is better to open extra registration after 10 minutes.

When registering during the extra time a participant is automatically assigned to a random room (among suitable for its role).