Today's mathforces contest screwed me over. Does anybody have good material suggestions so that I can quickly improve my skills in number theory and combinatorics?

Thanks!

I've seen blogs that talk about how you can earn over 200 dollars by creating Div 1, Div 2, or Div 1+2 rounds, however what about Div 3s or Div 4s? Thanks!

I'm trying to solve this problem: Xum

And it's hard enough that I look at the editorial.

Then I meet this Bezout's Theorem for the first time ever in my life, which online, it states that there exist $$$a$$$ and $$$b$$$ for any equation in the form of $$$ax+by=gcd(x, y)$$$, or in the case of this problem where both $$$x$$$ and $$$y$$$ are relatively prime, $$$ax+by=1$$$.

This is a classic Diophantine equation, which can be solved using the Extended Euclidean Algorithm. However what the problem asks us for is to find values of $$$a$$$ and $$$b$$$ such that $$$a,b ≥ 0$$$ and $$$ax-by=1$$$. How do we do this?

Thanks!

If you are like me and missed some tiny important details when reading, it might help to know beforehand that $$$x ≤ 999,999$$$ and is odd for all testcases.

Update: Thanks a lot to Thalleous and _inferno_ for their help! Please feel free to try and solve this if you haven't yet, and here is my submission if you want to check it out! 277308913

This contest, I surpassed myself by performing at 1804 level(as of now) with +84 delta according to the Carrot extension. However, I'm not sure if this will be enough for me to reach expert, as there were so many cheaters that I dropped by 300-400 in just the last 20 minutes of the contest... Will someone let me know? Thanks!

Edit: I did not reach expert. Time to wait for rating-rollbacks.

Edit 2: Yay!

for breaking the law(copyright) 2 times + reaching -100 contribution.

As I was scrolling through past problems today, I found myself thinking: This problem is wayyyy too easy for xxx(x).

For other CP websites such as Luogu, I know that problem difficulty tags are updated as the years pass in order to be more consistent with the modern difficulties(e.g. a 2100 problem back then may only be a 1800 or 1900 problem in the current times).

What about CF?

?????????

It's been 10 days, and the problem ratings for Round 959 still haven't come out :/

I'm not a native Chinese speaker(anymore), and my strong suit isn't in reading+understanding Chinese. Anybody who has used Google translate knows that it's kinda.... for translating problems(you get my point). Anyways, to those who are fluent in both Chinese and English + are skilled, please help me by providing Google Translate+ solutions to these questions. Thanks!

1. Mayan 游戏
2. 华容道
3. 消棋子
4. 数独2

....that involve LCA, MST, Treap, Sparse Table, Binary Manipulations and other advanced techniques.

Trying to build a good foundation. Thanks!

The Global Round will be my last chance to try and hit expert this summer! I'll be heading back to my home country China after the contest in order to have fun and improve my coding skills. I hope everyone reading this blog has a great summer and help support me by upvoting! :)

P.S. I'm still in the CM by September challenge! :D

UPD 1: I don't know why I got post-banned :/ so all my replies were deleted, but I appreciate everybody's support!

UPD 2: I was not able to reach expert, so maybe next time! :)