Do you already have a New Year's mood?

And we have traditional gifts!

Change Handle Feature

Hurry! Only until the 10th of January, you can change your handle (but only once)! Note that it will be possible to roll back the changes or change the handle again only after a year. Be careful what you wish for.

You can change your handle to the new one which wasn't used before by anybody or which was used by you before. The links to a profile page with old handle would automatically redirect to the actual profile.

This year we have an improvement. If you took part in at least 10 rounds you can request handle of an inactive participant. It means that the participant should have a period of activity on Codeforces of at most 90 days, this period should be in 2015 or earlier. The inactive participant can’t have posted comments, messages and so on. It can't take part in more than 2 contests. It will be automatically renamed and informed by email. If you can’t change handle to others, it means that some requirement doesn't meet. Please do not ask me to do something with it. I'm not Santa Claus.

Talking about handles I always reminisce the following story. Once a user wrote me the message: "Please change my handle from I_love_Valya to I_love_Sveta, as I no longer love Valya ..."

New Year's Masquerade of Colors and Ranks

Traditional magical tab has appeared in the profile setting. Happy New Year!

Hello,

We have some technical issues for operating with recent problems. If you created a gym/mashup (today or yesterday), it is possible that its submissions are not visible. I'm working on it. Probably, today night it will be a planned maintenance shutdown to fix it.

I think it will not affect Educational Codeforces Round 55 (Rated for Div. 2).

Mike.

Here are merged results of Mail.Ru Cup 2018 Round 1, Mail.Ru Cup 2018 Round 2 and Mail.Ru Cup 2018 Round 3 according to the GP100 scores (see the announcement for the details https://codeforces.net/blog/entry/62355). Best two contests give the summary score of a participant. This table is unofficial yet. But anyway... congratulations to the winnerzzzz!

Hello.

As I wrote into a comment, last round we are faced with a strong DDOS-attack which ruined the competition. I don't know who did it, I also don't know reasons to do it. I'm very upset about the situation and ready to make an effort to be prepared for such issues.

I spend a lot of time to be ready for such incidents.

Here are steps you need to do to be ready for unexpected failures:

• Join our telegram channel by the link https://t.me/codeforces_official to read urgent news.
• Be sure that you know the password of your Codeforces account. If you don't remember it, just use the password recovery feature. Please, do it right now.
• I've implemented a minimalistic website for replacing the main site in case of emergencies. Now you can only read problems, view your submissions (without any details), submit codes. Probably, later I'll add some more features, but anyway, the minimalistic version will have only vital features to take part in a contest. I've deployed it in several places, you can visit any of them by the links: http://m1.codeforces.com, http://m2.codeforces.com, http://m3.codeforces.com. If any of them is unavailable, just use another. Do not use them if the main website is alive.

UPD: Now you can enter minimalistic websites without a password. In this case, an email with a secret enter link will be sent to you.

Hi, Codeforces!

I am glad to announce and invite you to the second launch of my course on algorithms and data structures.

On 7-25 of January, 2019, I will be giving the course "Advanced Algorithms and Data Structures" at Harbour.Space University (Barcelona, Spain). It will be in English, and is not limited to Harbour.Space students — anyone is welcome! Who wants to join?

This course isn't just for Harbour.Space students, it is also open to Codeforces participants, who will be offered a special price, 1000 EUR. The cost does not include travel or accommodation.

The curriculum will include a breakdown of algorithms and data structures, a lot of practical exercises, and emphasis not only on the correctness, but also the beauty and structure of the code. My goal is to make classes that are useful and interesting for both those who want to understand the fundamental CS, and for those interested in programming competitions. And of course, we will have the opportunity to meet and talk. I look forward to share stories about the history of Codeforces and future development plans.

The course will consist of three weeks of intensive training, 5 days in each week, 3 hours per day. The program includes daily lectures and practical exercises. It will not be boring for sure!

Here is the expected course outline:

Harbor.Space University is located in Barcelona (Spain). For users of Codeforces, Harbour.Space is known for active participation in the life of the community of sports programming (partnership with Codeforces in the framework of Educational Rounds). The main activity of the university is teaching (there are bachelor's and master's programs) in the following areas:

• Maths as a Second Language
• Computer Science
• Data Science
• Cyber Security
• Interaction Design
• Digital Marketing
• High Tech Entrepreneurship
• FinTech
• BioTech
• Aerospace Engineering
• SuperCities UrbanTech

Harbour.Space is also proud to announce the Scholarships to study Msc in Robotics in Barcelona. Follow this link for more information about the opportunity and application process.

Register for my upcoming course via this link.

— MikeMirzayanov

Here are merged results of Mail.Ru Cup 2018 Round 1 and Mail.Ru Cup 2018 Round 2 according to the GP100 scores (see the announcement for the details https://codeforces.net/blog/entry/62355).

Hi Codeforces.

I am glad to share a small but useful update of Polygon, which was fully developed by me in the walls of ITMO. Now preparing problems with unusual I/O will become a little easier.

Now in the new problems you will get examples in the statements without any transformations by LaTeX/HTML. If earlier you had difficulties with the correct formatting of empty lines or the fact that a double hyphen is replaced with a dash, now there are no such difficulties. The enhancement works for both PDF and HTML statements.

For example, to have such I/O examples just add such a test and use the appropriate output from the model solution.

Note that the feature to overwrite the examples is stayed working (custom content of input or output data for statements). It seems that there are almost no reasons to use it for an input now (apparently, only for interactive problems).

Previously created problems use the old approach, so this innovation should not break existing problems.

How do you like the feature?

Hi Codeforces!

Meet a small innovation on Codeforces — difficulties of problems (and at the same time a new widget filtering problems in the archive). For all the problems of the archive, I’ve calculated the difficulties in the scale of the rating of participants. Approximately this means that if the rating of the problem is equal to yours, then on a typical round you would solve the problem with a probability of 0.5. And, in general, if your rating is r_i, and the problem rating is r_j, then the problem during the round can be solved approximately with probability:

For example, if the rating of a problem is less than yours by 200, then the expected probability of solving the problem is 0.75. With a difference of 400 rating points, the probability increases to 0.9.

For convenient search of problems in the archive, you can now use a special widget:

With it, you can find not only problems that have all the chosen tags, but also which have at least one tag from the list.

A difficulty of a problem is also displayed when choosing problems in a mashup.

I hope that now you will be able to do more effective practice, and the process of making new mashups for trainings will become easier.

UPD 1: Have you already noticed new pop-ups about judgment verdicts of your submissions?

UPD 2: (API update) Optional field rating has been added to the object Problem in API.

Hello!

ACM-ICPC Southern Subregional Contest (NEERC/Northern Eurasia) 2018 has ended on October 16. There were 72 teams onsite in Saratov, most of them were invited because of their result on the qualification stage.

On Oct/20/2018 11:05 (Moscow time) will start online-mirror 2018-2019 ICPC, NEERC, Southern Subregional Contest (Online Mirror, ACM-ICPC Rules, Teams Preferred).

In this contest I play a role of Cheif Judge and the jury teams consists of ex-participants of ICPC from Saratov and jury members from other cities. Many thanks to all of them! I hope you will like the problems!

I invite ACM-ICPC teams and individual participants of Codeforces competitions to take part! Sure, the contest will be unrated.

MikeMirzayanov

Hi Codeforces!

For you, perhaps it was yet another Codeforces round. But not for me. Codeforces Round 515 (Div. 3) is the first round tested on new judging servers at ITMO University. And this is not just an update of location. Ta-dam! Now your solutions will be judged on the new Intel i3-8100 processors. And this is not all news. The number of judging servers has increased, which means fewer queues during rounds!

I am pleased to announce that now I live in St. Petersburg, I work at ITMO, and Codeforces is gradually moving from the walls of dear to me Saratov University to ITMO University. The decision to move was not easy for me. My plan is that, based on ITMO, I can focus more on Codeforces development and work on the platform. The number of world champions per square meter is simply overwhelming, and working with a large team of such enthusiasts (and professionals!) of sports programming, like me, is extremely inspiring. I always liked St. Petersburg and the atmosphere of ITMO. Intuition did not let me down. I feel surrounded by people close to me in spirit (and I’m not only talking about a work). I am sure there are many interesting common projects ahead!

I do not say goodbye to Saratov. This is my hometown, full of people dear to me. I came to my first programming training at SSU exactly 20 years ago. Antonina Fedorova, thank you very much. Natalya Andreeve, I would like to say a personal thank you now. You have opened for me an interesting world of programming competitions. We were happy together when we first advanced to the ICPC Finals, and later when we became champions of Russia and the World. We made countless competitions and helped many Saratov students find themselves in programming. I fervently support the future of the Programming Competitions Training Center at SSU and future generations of Saratov contest participants. And now, I am in Saratov and still the head of the jury of the ICPC Subregional Contest, and even an SSU employee. I hope that we will make a good and interesting contest.

I will try to make a complete relocation of the Codeforces infrastructure to ITMO without downtimes. A good Internet connection between SSU and ITMO is encouraging. All the planned work will adapt to the schedule of the rounds, and now it pleases more than ever (I send my greetings to the coordinators!).

Currently, all Codeforces and Polygon solutions are being judged on new servers based on Intel i3-8100 processors. Fortunately, the performance of a single core is not very different from the one that the old generation of judging servers had. Thus, the time limits in all problems remain the same.

Such news. I am waiting for you on Codeforces Round #516 (by Moscow Team Olympiad).

— MikeMirzayanov

My friends from Harbour.Space and Remy Robotics asked to publish the news. I am excited and glad to do it, because for our community this is a true exclusive offer. In short, they offer to study robotics in Barcelona, paying for your studies and even with a scholarship! It's so cool!

Here is a direct speech from Harbour.Space.

=

Hi Codeforces!

We are excited to announce our new Master’s in Robotics programme scholarship, which will be paired alongside an internship with our partner Remy Robotics! The programme will begin on January 7th, 2019, at our university in Barcelona, Spain.

Harbour.Space’s Robotics programme is the bridge between a personal interest in the world of Robotics, Artificial Intelligence, and a top-level professional future in one of the most exciting and fastest growing fields of technology. Students who enter the programme will either graduate as Control Engineers (theoretical specialists who ensure that robots interact with the environment in safe and effective manner), or CV Engineers (industrial manufacturers of robots, based more on practical experience).

Students will learn the Design and Control for Dexterous Manipulation; Kinematics, Dynamics and Control; Advanced Manipulation Algorithms robots use to physically interact with their world; Dynamic Optimisation for behavioral control; Integrated Intelligence in Robotics: Vision, Language, and Planning which builds upon the cognitive development; Mechanics of Manipulation focusing on using intelligent development of kinematic constraint, gravity, and friction; Manipulation, Estimation, and Control allowing for robots to locomote and navigate the world; Reinforcement learning in Robotics.

SCHOLARSHIP VALUE: €34,900

The scholarship includes:

• Complete coverage of the University tuition fee (€22,900)

• Living allowance (€1,000 per month during 1 year)

• Internship at Remy Robotics (20h per week during 1 year)

REQUIRED EDUCATION:

Bachelor's or Master's Degree in Robotics or Computer, Electrical, Mechanical Engineering or related disciplines.

QUALIFICATIONS AND SKILLS:

• Hands-on robotic programming
• Ideally experience within the automotive manufacturing sector
• Knowledge understanding of robot control interface with ancillary equipment
• Use of robot simulation packages
• Deep experience with all things robotic, from infrastructure-free autonomy to ROS, computer vision, and machine learning
• Experience working with robot parts and components, developing robotics devices
• Ability to concurrently manage multiple diverse and often complex issues and / or projects at the nexus of software, sensors, and hardware

To be selected for this programme, you will need to go through the following steps:

1. Fill out the given form by the link: https://codeforces.net/userForm/89f420923cb55373
2. Attend a series of online tests and interviews with our admissions office and partners
3. Pack your bags for Barcelona!

Problem idea: MikeMirzayanov, awoo; prepared by: vovuh.

Problem idea: MikeMirzayanov, prepared by: MikeMirzayanov.

Problem idea: MikeMirzayanov, prepared by: awoo.

Problem idea: vovuh, MikeMirzayanov; prepared by: awoo.

Problem idea: Errichto, prepared by: Errichto.

Problem idea: Lewin, prepared by: Lewin.

Problem idea: Endagorion, prepared by: Endagorion.

Hello, friends!

I want to discuss separately the negative feedback on the round 505.

I understand, that many participants are disappointed by solutions, which failed on system tests. In fact, the pretests in problems B, D and E turned out to be weak. The problem C also didn’t gone too smoothly, but I don’t see nothing critical.

Surely, this was a flaw in a work of author and coordinator. Unfortunately, such situations sometimes arise and it is difficult to avoid them at all. For these problems, the number of pretests and their type was not looking too weak.

Problem B: 9 pretests, there are small and large answers, two tests with answer -1, there are pretests with n = 1 and n = 2, there are four pretests with n = 150000.

Problem D: 14 pretests, among them manual tests and four different generators, few pretests with n = 700, majority of answers is ‘Yes’, but there are ‘No’ as well. In my opinion, too little pretests with ‘No’.

Problem E: 14 pretests. Yes, this problem on VK cup finals contained 10 pretests and caused many systests fails for participants. I have added 4 more tests to pretests from tests, which caused system tests fails for onsite participants. I was very surprised to see, that there were still so many fails after systests.

Summing up, the pretests turned out to be incomplete, but it is hard to say, that it was obvious defect by author or coordinator. Probably it is a combined effect from the problem specifics and the lack of experience of cdkrot as a coordinator.

I haven’t examined all the problems thoroughly, but still round seemed interesting to me. There were no serious fails with statements, bugs in tests and solutions. The system was also working smoothly, without large queue.

Please explain your negative feedback about the round. It will be very valuable to read a reasoned opinion from experienced participants.

Thanks, MikeMirzayanov

Hello!

I am happy to sum up the results of the draw of prizes in two rounds. Congratulations to the top-10 with the win of a plush Peach!

Hello, Codeforces.

This is a short blog to introduce you recent updates in Testlib and Polygon.

Testlib

Generate a Permutation

Now, you can easily generate a permutation with codes like this:

Function println

Now, you can easily print space-separated lines in a generator. A println uses cout, thus prefer faster method if you print huge data.

Some examples:

Here is the example of a generator to print a permutation:

#include "testlib.h"

using namespace std;

int main(int argc, char* argv[]) {
    registerGen(argc, argv, 1);
    
    int n = atoi(argv[1]);
    println(n);
    println(rnd.perm(n, 1));
}

Function readInts and similar

Just as a reminder. Use functions readInts/readLongs/readStrictDoubles or readTokens/readLines in a validator to read and validate a sequence of values. To read a size of an array and array itself, use:

int n = inf.readInt(1, 200000, "n");
inf.readEoln();
inf.readInts(n, 1, 1000000, "a");
inf.readEoln();
inf.readEof();

Polygon

Example Problems

I've introduced three example problems. Each Polygon user has READ-access to them. Please, use them as examples how to write a problem in Polygon. They are:

• example-a-plus-b: simple A+B problem
• example-almost-upper-bound: simple problem to illustrate non-standard checker (always consider to use readAnswer function like in the example), generators and stress tests
• example-interactive-binary-search: simple interactive problem on a binary search

Other Small Fixes

• Allow to upload files (for example, images) as a contest property/file to use them in the statements.ftl. File names should start with 'statements-'.
• API has been improved to support general description, general tutorial, tags, test groups and points.
• Show problem ID on the summary box (on the righmost top block).
• Replace UTF-8 typographic characters to their ASCII equivalent (for example, replace em dash — with ---).
• Caching issue has been fixed. Previously, it could show RJ on tests even if the reason has been fixed.

Thanks to fcspartakm for implementing most features in Polygon.

Hello, Codeforces!

As many of you I'm not really satisfied how the round 497 has ran. KAN is not working on Codeforces this month, he is out of the city. It was a first round coordinated by arsijo. I liked his enthusiasm and work he did. But it happened, it wasn't an example of a well-prepared round.

The main issues are as follows:

• The problem statement in the problem A wasn't clear enough (actually, it didn't contain any mistakes, just wasn't easy to understand).
• The improper example to illustrate the problem B (no mistake here, but it has confused some users).
• I really didn't like the statement of the problem D2D/D1B. I agree it wasn't good. Again it didn't contain any mistakes, but it was hard to understand it quickly. Also I did the incorrect announcement (but it was fixed quickly).
• The incorrect behavior of the interactor in the problem D2E/D1C (Time Limit Exceeded/Idleness Limit Exceeded on too many queries instead of Wrong Answer).
• The huge gap in the difficulty between the D2C and the D2D.

Right now, I'm really do not sure, should the round be a rated or not. What is your opinion about it? The current idea of the writers and the coordinator to make it unrated for whose, who got incorrect verdict on D2E/D1C.

Anyway, I'd like to say “thanks” to the problem writers, testers and the coordinator arsijo. They really tried to make a good round. I hope to see more rounds from Skyglow and coordinated by arsijo. I'm sure they made proper conclusions from the round.

Thanks, MikeMirzayanov

UPD: Thank you for sharing your opinion. The round is rated.

Hello!

This time decided to fill myself in the shoes of the problem writers. It is very exciting! My challenge was to prepare a round in one day. It's really incredible pleasure to surrender to my passion and all day just work on problems!

Despite the fact that in total I've wrote 8 problems, I made it in time. Initially, I prepared 7 problems, but two of them were found to be used before (thank you, 300iq and cdkrot for poining it) and I removed them and wrote a new problem.

Codeforces Round 496 (Div. 3) will start on 09.07.2018 18:35 (Московское время). You will be offered 6 problems with expected difficulties to compose an interesting competition for participants with ratings up to 1600. Probably, participants from the Div. 1 not be at all interested by this problems. And for 1600-1899 the problems will be quite easy. However, all of you who wish to take part and have rating 1600 or higher, can register for the round unofficially.

The round will be hosted by rules of educational rounds (extended ACM-ICPC). Thus, during the round, solutions will be judged on preliminary tests, and after the round it will be a 12-hour phase of open hacks. I tried to make strong tests — just like you will be upset if many solutions fail after the contest is over.

You will be given 6 problems and 2 hours to solve them.

Remember that only the trusted participants of the third division will be included in the official standings table. As it is written by link, this is a compulsory measure for combating unsporting behavior. To qualify as a trusted participants of the third division, you must:

• take part in at least two rated rounds (and solve at least one problem in each of them),
• do not have a point of 1900 or higher in the rating.

Regardless of whether you are a trusted participant of the third division or not, if your rating is less than 1600, then the round will be rated for you.

Many thanks to the testers: kevinsogo, 300iq, cdkrot, arsijo and adedalic. You really helped to make this round!

Good luck!

UPD 1: The round is over. Thank you for participation!

Official Top-5 (trusted only)

• poteryan_i_ne_naiden
• num2
• qaweqa
• HCMUS-Thanos
• tantam75

Unofficial Top-5 (+ untrusted)

• BigBag
• kmjp
• quailty
• TangentDay
• Aeon

UPD 2: The editorial is available by the link.

Hello, Codeforces!

I offer you also to join testing of HTTPS support. Just visit to https://codeforces.net/ and start to use the website through HTTPS.

I am aware of some minor issues: mails still refer to the HTTP version of the website, Facebook does not merge likes for HTTP and HTTPS. Have you noticed anything else? Write a comment!

And thanks to Let's Encrypt for the certificates. Now with wildcards!

Hello!

Codeforces Round 481 (Div. 3) will start on May/13/2018 12:05 (Moscow time). It will be the second Div.3 round in the history of Codeforces. You will be offered 7 problems with expected difficulties to compose an interesting competition for participants with ratings up to 1600. Probably, participants from the first division will not be at all interested by this problems. And for 1600-1899 the problems will be too easy. However, all of you who wish to take part and have rating 1600 or higher, can register for the round unofficially.

The round will be hosted by rules of educational rounds (extended ACM-ICPC). Thus, during the round, solutions will be judged on preliminary tests, and after the round it will be a 12-hour phase of open hacks.

Remember that only the trusted participants of the third division will be included in the official standings table. As it is written by link, this is a compulsory measure for combating unsporting behavior. To qualify as a trusted participants of the third division, you must:

• take part in at least two rated rounds (and solve at least one problem in each of them),
• do not have a point of 1900 or higher in the rating.

Regardless of whether you are a trusted participant of the third division or not, if your rating is less than 1600, then the round will be rated for you.

The problem are written by me and fcspartakm. Many thanks to testers AGrigorii, BigBag, nhho and Sert!

Good luck!

UPD 1: Thank you for participation. Problem tutorials have been published.

UPD 2: Congratulations to the winners! Top-5 (official standings):

• AntonD
• pedulia
• liyingyan7
• luhong
• Silver-Water

Hello, Codeforces!

Recently I've started to receive a lot of messages with the ask to delete an account. I'm thinking how to implement this feature. What content should be hidden after it: posts? comments? solutions? How it implemented in Facebook and other social networks? Are there any generally accepted norms about account deletion?

Waiting for you ideas in comments.

Hello, Codeforces!

It's time for news. I hope you will enjoy!

• We reduce the rating bound for the orange color. Thus, now the orange rating (and the Master title) will start from 2100 rating units.

• The lower bound of the rating for legendary grandmasters is increased to 3000 rating units. Thus, right now in the community, 14 participants meet this requirement. Real legends!

• For rounds like "Div. 2 Only" the upper bound of the rating for official participation is increased. Now everyone whose rating is less than 2100 (i.e., before masters) will be able to register for such rounds to participate officially, their rating will be updated based on the results of the round. We do not plan to increase the complexity of Div. 2 rounds. Practice shows that the problems of such rounds are interesting and useful for participants "below the orange".

• For parallel Div. 1 and Div. 2 rounds we don’t change the rating bound between divisions. So Div. 1 edition of the round continue to include participants with the rating from 1900 and above.

• As an experiment, we plan to host rounds of simplified complexity (Div. 3). In such rounds, the official results will include participants with ratings less than 1600. We plan to include in these rounds simple training problems that will help beginner participants to gain skills and to get new knowledge in a real contest. Since even in the Div. 2 rounds regularly some Div. 1 users register new account and take high place, we also expect such unsportsmanlike behavior in the Div. 3 rounds. We will exclude from the official standings of Div. 3 rounds and put in a separate rooms all those who can’t be reliably called to be a real participant. Accounts that materially participated in less than 2 rating rounds (materially means solved at least one problem there) before the start of the Div. 3 rounds, and those who have ever gained 1900 or more rating units will not get into the official standings and will be assigned to separate rooms. However, this does not mean that there is no rating recalculation for them. Thus, the rating will be updated for all users whose rating is strictly less than 1600 at the time of the start of a round. Perhaps we will experiment with the definition of who should be considered a reliable Div. 3 participant. And thanks to VLamarca for the post about idea of Div.3 rounds!

Updating the boundaries of ratings and changing colors is not an instantaneous process. For this reason, the website may still have outdated information for some time. We are working to apply the announced changes during the day.

I hope you enjoy this news.
— MikeMirzayanov

Hello!

This blog post is about the end of the crowdfunding campaign dedicated to the 8th anniversary of Codeforces. Everything worked out. Hooray!

For us, it is very important to support not only in terms of the collected budget, but as a powerful indicator that Codeforces is important to you. It was very pleasant to read words of gratitude and congratulations. Thank you!

We received more than 1000 donations from community members from around the world. For 45 days we've collected more than $60000 dollars. The collected funds will allow us to continue to move forward, to please you with new contests and improvements of the system.

I am happy to report that starting in May 2018, we will be able to slightly increase rewards for problem writers.

We tie the money paid in rubles to the Central Bank of Russia exchange rate, rounded to the closest sum in rubles divisible by 5 (according to the rules of mathematical rounding). The table shows the values that were up-to-date when this post was published. An asterisk marks the bonus that is given if the round is prepared in time and without major issues in problems.

In addition, the collected funds will help to increase a reward of the problem coordinators KAN and [user:gritikan]! Guys, you're doing a very important job, thank you for that!

Improvement of Codeforces hardware has already been launched, which will help improve the stability of the system and keep up with the growth of the audience.

To all those of you who are expecting a gift from us (certificate, T-shirt, hoodie), I remind that it is necessary to fill in or update the information in the profile about the delivery address. Please do not delay this!

Thank you again for your support!

MikeMirzayanov

April 19, 15:35 (UTC) the VK Cup 2018 - Wild-card Round 2 will start.

Participants are invited to achieve progress in solving an unusual problem. VK Cup teams which were advanced to the Round 2 (and didn't advance to the Round 3) will take part in VK Cup 2018 - Wild-card Round 2 officially. In addition, this round will be open to the public for unofficial participation for everybody. Registration will be open for the whole round duration.

The round will be one week long. After the end latest submission (with positive score) of each participant will be judged on system tests.

Good luck!