Can you people please stop peddling these useless paid course to newbies.

This is not necessarily a mistake. If most of the people who got DQ'd were people you beat, that means your performance is less impressive from an elo perspective with those people removed.

It is a good point though that getting caught for cheating is perhaps not a severe enough penalty. Something that could fix this and curtail cheating long term would be if everyone who is DQ'd scores 0 on the contest instead of being removed. Then your rating would only increase when people are removed, and it would discourage cheating more (because if you cheated and were caught, now your rating would suffer), and it would also encourage people to find cheaters and help improve the system (because doing so would improve their own rating).

I don't think track and field athletes are churning out integrals before going to some race. Rathermore, the mental training you're talking about is about training their minds to not get nervous during their most important moments, a thing really embedded within all aspects of human life, or particularly already within such competitions. As such, I feel it is clear that in this situation, this sort of preparation, (however generally beneficial to the contestant outside their competition lives and such), brings with itself an additional unnecessary and tangential burden.

Furthermore, for people localised within certain low countries, this burden can easily become a financial one, considering that for full acclimatisation preparation, one would probably have to travel to far away mountain ranges or buy heavy machinery to simulate such processes. This sort of stuff, I feel, goes far beyond what the scope of IOI should be.

Yeah, ikr. I really wish he'd post more.

All committees travel to the IOI venue for a week half a year before IOI. If half of us will be unable to make it to the meetings, I hope we'll notice.

Unexaggerated version of this is a valid concern. And although there isn't a place where everyone will be in the same conditions (time zones being the most basic example), I'm sure that potential mitigations of this will be discussed in-depth with the hosts closer to IOI 2025.

Cf peak: 1791 Chess peak: 2411 blitz (chess com)

No, dude, that's straight up silly

UPD: My video covering my chief-judging experience is available here. Apologize for the long delay.

Cool video, thanks!

CF:2315, chess.com:925

The theorem can be found on this Wikipedia link. It says $$$a^p \equiv a \mod p$$$ where $$$p$$$ is a prime number. Thus you can think of the fact that $$$a^{p - 1} \equiv 1 \mod p$$$ and then $$$a^{p - 2} \equiv a^{-1} \mod p$$$. Thus by multiplying by $$$a^{p - 2}$$$ you essentially divide by $$$a$$$. To calculate $$$a^{p-2}$$$ fast you can use some version of fast exponentiation algorithm.

No.

We always hold offline contest and our teacher judges the code using his test cases. We can't get the test cases so after the contest ends, we can't test our codes again.

So I need somewhere to test my code.

I am very bad in chess

We could say such thing about the bank account as well, but I doubt your reaction would be the same

CodeForces: 2872 (recent peak: 2908) / Chess.com: 987 (Rapid)

Codeforces peak: 2887 Chess.com rapid peak: 1569

AlgorithmsThread 1: Division Under Mod!

This was a fun watch! Thanks for sharing your experience.

thank you for the problem E, for the first time I felt that I was not studying mathematics for nothing, when I realized that Wilson's theorem was applied there, sorry I didn't think of two difference arrays, but still thank you very much for the problem

i feel like hacking should just be completely removed.

There is no current purpose to it, 99% of hacks are not some smart case tripping up wrong greedies but a string hashing hack or umap hack etc.

if you have rooms (even after contest), it is a matter of luck (imagine losing 1st place to this, yes it has happened), if you do not have rooms, it is even more a matter of luck and just finding the wrong guy fast.

• CF peak: 2324, now 2181
• Chess.com peak (5+0): 2122, now 2036
• Lichess.com peak (5+0) : 2200, now 2142

CF peak — 1745 chess.com rapid peak — 1665

Wow. Obviously I cannot solve E within the contest, but I managed to get the correct approach after 1 hour of thinking yesterday and 30 more minutes today. It's at least 3 twists. I didn't know Wilson's Theorem.

But I did it without the editorial!

thx! idk that "plag" means plagiarized. :)

thank you! actually I just found that my ranking in last cf round has changed.

can somebody tell me what is "plag check" and what has happened(I can't find this in recent actions......)? I found my rating has increased 6 or 7

looks like leetcode copied your problem

these two again got undetected by just using some variables which are of no use 257640596 257641772 , i already mentioned about this in the contest blog. Vladosiya. They are doing this from past contests too. (https://ibb.co/TTQGHV3)

You read uninitialized values from sz so your behaviour is undefined.

Yeah lmao. When I previously reported some cheater, some user told sth similar also,

went to specialist before plag check but now my rating has dropped by 20 points:(

Wow, it's more than twice the height of Denver. As NBA fans, we know how intimidating it can be to play away games in Denver.

Thank you for the great cheatsheet!

I personally don't see those mistakes as a serious problem at all. In fact, even experienced people also make mistakes. For example, while $$$1$$$ is a prime number (since prime numbers are the numbers that are only divisible by $$$1$$$ and itself), people sometimes mistakenly think it is a composite number (composite of what? LOL). Sites like Library Checker make the same mistakes as well.

But since number theory is a relatively undeveloped area of math, it is understandable.

MikeMirzayanov could you look at that issue please?

CF:1280 Lichess:2008

Anti-cheating system to disqualify or penalize cheaters. So basically, when cheaters are penalised or disqualified, Elo changes because your relative performance changes and that's why rating changes.

Just keep solving problems, and once you get to the problem which you couldn't solve because you didn't know some technique, learn it. But always remember to learn technique fully before going to another one, like i somehow went from trying to learn parallel binary search to hamiltonian path and then to persistent segment tree

Ayy, that's great! Discovering all the twists when coming up with the problem was soo fun, it honestly the best part about this problem imo. Hope you enjoyed solving it!

Roadmap:

• 10min after contest: worked out the formula of the answer, thought this is a high school math problem because $$$\sum_{i=m}^{n} C_i^m = C_{n+1}^{m+1}$$$ but later found the $$$\bmod j$$$ cannot be processed this way
• 20min: realized the result is $$$0$$$ for composite j except $$$4$$$ and proved it
• 35min: guessed Wilson's theorem with observations from $$$3$$$, $$$5$$$, $$$7$$$ and $$$11$$$, then Googled it out (Twist 1)
• 50min: Confirmed that the answer for each j starts at $$$-1$$$ and -- once every j elements, cyclic-wise. (Twist 2) I was confident enough that I can code it and went to sleep.
• Today 0min: learned Euler's sieve. I should have learned it earlier.
• 10min: began coding but soon found sum of $$$n$$$ is unbounded and a bunch of TLE2 solutions! Start looking for offline method.
• 30min: Realized I could generate duplicate elements with prefix sum, and the answer with another pass of prefix sum. (Twist 3) Time $$$O (n*\sum_{p is prime}^{p\le n}1/p)$$$.
• 45min: AC first try.

cf — 1770
chess.com — 0 :3

This olympiad in my opinion had not the best problems and not the best TL(hate day 1 A,B, and TL on day 2 A), but I had some fun.

On day 1, I spent 4h out of 5 just grinding C to get AC using matrix exponentiation (got 81, optimized to around 0.9-1.2s, but TL is 0.7, which makes sense), and on day 2 spent 4.5h out of 5 just getting AC on A (really wanted to get full score, and quickly knew how to solve it), because TL was very strict for me (in fact I almost gave up, but at the end resubmitted my solution, which was previously working 1.012s to 0.994s).

Funnily, those two problems (81+100) were enough to get me silver (plus 27+7 on other two problems), so that's cool. I think I will take it more seriously next time, not grind one problem for the whole contest, and try to get much higher place.

Cf ~1600

Chess.com ~700 (but I haven't played for a long time and now my level is much lower)

Can you link the account ? Maybe, it's a newly created.

I think that's because you had participated in no more than 5 contest, so the rating changes are rapid. Lucky you :)

The system skips submissions of cheaters and people who plagiarised solutions.

I had something similar to me (but much less change) — I was at 1524 and now I am 1599. I am a little thankful since I can get in one last Div. 3 (hopefully)!

During the last recalculation, an issue occurred. We are working on identifying the cause and fixing it.

Thank you!

You formulated a linear programming problem (and then solved its dual). Do you understand what happens if we try to solve any of the primal/dual using, for example, the simplex algorithm? Can it happen that it will do exactly the same as we do with arbitrary pivot rule? Because increasing something and decreasing something else while something holds looks an awful lot like moving along an edge of a polytope until we reach another vertex. If that turns out to be the case, it will add another layer of abstraction, which is nice for understanding the structure.

I wouldn't say that I studied chess extensively but I definitely tried to do my best for quite some time, and I am just struggling to climb higher. I guess it is just a different skill... I got to this 1300-1600 range almost immediately, and then I am not progressing at all.

hello , i am state chess champion { username --> chess.com = laxmi_harii, lichess = laxmi-hari }

In my experience every time it happens, delta increases by +1 or a little above. But, this is the first time to decrease. Not by -1 (I wished). It decreased (for me) by -34 points!

Also, for my expert friend (previously hh), it is his first time to encounter something like that.

Dude, he stole our rating))

I hate the sicillian defence

What is your question though?

CF peak: 2391 Chess peak: 875 (chess.com)

Am I the only one who feel like chess is impossible to learn???

P.S. Heartbreak for Fabi :(

Sorry for the downvote, it was by accident. I do want to upvote.

Could someone send me some problems like D please.

Hi, great catch! We had written this formula with $$$i$$$ being 0-indexed, as it was in our code. The formula you gave is correct when $$$i$$$ is 1-indexed, which would be consistent with the rest of the solution.

CF — 2318

chess.com — 1634 (blitz)

useless blog

Im at almost +200 after the check. Guess it has smth to do with the fact it's one of my first contests.

Yes, I saw couple of newly created accs. I found CM in 4 contests, many expert in 3 contests.

Seems like freshly created accs stole old ones rating xD

Here we go

sebak anta mn el 7awar da el manga el3wesy 3amla kam 3andoko

CF: ~2150, lichess: 2000

It's magic like vectorint_40000_7

• CF Peak: $$$2178$$$
• Lichess (Blitz): $$$1900$$$

Can anyone please refer more problems like C & D

This is one of the best rounds I have had in recent times, with clever ideas in every question! Hope to see more rounds like this~

I support! xD He is too dumb, not all skips mean cheating, a resumbission skips and causes -50, but maybe that's his experience in cheating xD.

Btw, orz Maxim!

No.$$$f(x,z) \operatorname{xor} a_y$$$ is not always greater than $$$f(x,z)$$$. Example:$$$f(x,z)=6$$$ and $$$a_y=2$$$.so that $$$f(x,z) \operatorname{xor} a_y = 4 \leq f(x,z) = 6$$$

I guess you mean that: if $$$a \geq b$$$, then $$$(a \operatorname{xor} c) \geq (b \operatorname{xor} c)$$$. It is obvious thar $$$(1 \operatorname{xor} 1) \lt (0 \operatorname{xor} 1)$$$ but $$$1 \geq 0$$$. Generally, the operator $$$\operatorname{xor}$$$ does not have associative laws as addition or subtraction. $$$(a+b) \operatorname{xor} c$$$ is not always equal to $$$a \operatorname{xor} c + b \operatorname{xor} c$$$. Hope to solve it. :D

Oh okay, Thanks.

WOW. Decent enough in both fields.

Just from curiosity, which field feels a bit complex and hard to you?

lichess.org , chess.com ---> lichess = 2200+ , chess.com = 2200+

Sum of the first n numbers is n * (n+1)/2 not n * (n-1)/2.

If n=5 1+2+3+4+5=15 then n * (n+1)/2= 5 * (5+1)/2 = 15 and n * (n-1)/2 is 5 * (5-1)/2 =10.

Also Number of ways to choose pair of two numbers from n elements =nC2=

(n * (n-1)* (n-2)!) / ((n-2)! * 2!)=n * (n-1)/2 not n * (n+1)/2.

Another Correction:

1) It's Binomial

2) Number of ways to choose two numbers out of n numbers: c(n,2) = n*(n-1)/2

Advice: Help yourself first before Helping Others. (As Every Flight Attendant ever said)

In math/math related problems you mostly encounter algebraic, rarely geometric concepts. First of all, you have to be familiar with number theory, know how numbers operate, be used in functions, how they get canceled in some cases. It's advised to know some basic formulas as $$$(a+b)^2=a^2+2ab+b^2$$$. And it's easier if you divide a task into subtasks.

Example problem: You are given an array $$$a$$$ of $$$n$$$ integers. You have to find some integer $$$m$$$, so that $$$f(m)$$$ is maximised, where $$$f(m)=(m\ mod\ a_1)\ +\ (m\ mod\ a_2)\ +\ ...\ +\ (m\ mod\ a_n)$$$

You start out simple, what is the maximum value of $$$m\ mod\ a_i$$$? It's $$$a_i-1$$$, when you set $$$m$$$ to some $$$a_i \times k - 1$$$, where $$$k$$$ is an integer

Furthermore, can you choose such $$$m$$$, that $$$m\ mod\ a_i$$$ is maximised for all $$$i$$$? Turns out yes! If you multiple all elements of the array and subtract one, so $$$m=a_1 \times a_2 \times ... \times a_n-1$$$ (However there is a simpler solution, where $$$m=-1$$$), and for every $$$i$$$, $$$m\ mod\ a_i$$$ is maximised

Lastly, when you fully done and understood the problem, you can cancel out unnecessary things, and the remaining part is to implement. In this problems case, the final answer is $$$sum(a)-len(a)$$$

Of course all problemsetters tend to hide the answer in testcases, where they give weird examples, so it's almost impossible to find any pattern fast. So, it's easier to check cases by hand and hopefully be able to recognise a consistent rule, pattern, working for all cases.

If you struggle with basic math concepts, then you could look for courses in any online resource, after all, internet is a free-source library, where you can find almost anything. Don't know/understand something? Google it!

I hope this one was helpful

Competitive programming is definitely more complex than chess.

As for which is harder, I'm not good enough at either to give a qualified opinion (definitely chess though).

2374 blitz (highest 2438)

Not a big achievement since I was learning chess since I was 7. I know at least two CPers who are better in both CP and chess.

Yeah I still keep solving problems, but I think I will stop doing contests in codeforces cuz it's not improve my algorithms a lot, which I weak at. And I think I should spend my time learning more algorithms to do more problems in each contest. Do you agree with this opinion?

@mihajlovskijr Thanks for your recommend! Maybe I will do the next contests to see the techniques have been used nowadays. But I need to learn more about algorithms cuz...I don't even know segment tree xD All I know is math,dp and some kinds of recursion and graph so...

Damn, really great problems. Its a shame i didnt know modularni inverse at the time but lnow i do.

Yeah a sudden Rk 5 and his template changing is definitely an indicator it wasn't him doing the contest.

For some reason it just fixed itself.

Seems fair 257640392 257630953

CodeForces Peak: 1463, lichess (Rapid) peak: 2002

насколько ускоряет?

It is codeforces not a platform like instagram It is coding site

Похоже это связано с тем, что сейчас пересчитывают рейтинг. По идее скоро всё должно вернуться в нормальное состояние.

Why last COdeforces Div2(Round 940) and codecraft-23 problems rating are not available in CLIST? (Earlier it used to show rating is pending but now, problems aren't visible)

My rating decreased by around 20 points. This is the first time rating got decreased during temporary roll back; it used to increase by 1 point before. Got something to do with last DIV 4 contest I believe.

At this point, there's gonna be no need for such a camp with these rigged rating changes for newcomers to Codeforces...

Any update on the prizes Fremder

Best div ever , even if you disagree B⁠-⁠)

Huge congrats to Gukesh for the win, although I'm heartbroken for Fabi. Anyways great (and quite surprising) to see chess come up in here!

Recent changes:

• 2024-22-04 PyPy 3.9.10 (7.3.9, 64bit) upgraded to PyPy 3.10 (7.3.15 64bit)
• 2024-22-04 Go 1.19.5 upgraded to Go 1.22.2

can someone please explain the second part of dp transition in problem c?

Edit: Doubt cleared.

I found logic easily for D but couldn't implement the solution (T_T).

My rating also decreased for 9 points for some reason