Hello, Codeforces community!

Today's contest was a nightmare for me. Let me tell you what happened.

First of all, I was 2 minutes late due to the networking issue. When I start the contest, 1000+ contestants already solved A. This made me extremely stressed.

I eventually calmed down, and passed both A and B in 15 minutes.

The next problem is C, which looks very trivial. However, I did not expect that I got stuck and could not figure out the solution in 20 minutes!

So I had no choice but to go and look at other problems.

My English is not that good. So I chose problem E, the one with the shortest problem statement.

I found it unbelievably easy and solved it in just 10~15 minutes.

Now I still have 75 minutes left.

I looked at D. Very easy, I can just use some STL (std::map and std::set) and binary search to solve it. However, it might took me a while (like 20~30 minutes?).

I looked at F. Seems not so difficult as well.

By the time, around 2000 contestants passed C. I thought "Maybe I overcomplicated the problem. I should be able to rush it." So I decided to work on C, which might be the stupidest mistake I made today.

I got stuck for another 40 minutes despite I already found out that x is equal to the greatest common divisor (GCD) of all the difference between adjacent elements.

20 minutes before the contest is over, I finally finished the code. However, since I initialized the maximum element of the array to be 0 (actually it should be -10^9 because there are negative elements), I received 3 WAs, which cost me lots of penalty.

I eventually passed C around 5~10 minutes before the end of the contest and then I had no time for D and F...

This is not the first time that I being destroyed by "easy" problems....

In Codeforces Round 903 (Div 3), I solved everything but C (which is a 1200 rating problem).

In Codeforces Round 907 (Div 2), I solved problem F (2000 rating problem) but failed to solve C (which is a 1500 rating problem), again.

In Codeforces Round 910 (Div 2), something even crazier happened. I solved D (1900 rating problem) and E (2200 rating problem) while failed to solve both B (1500 rating problem) and C (1700 rating problem).

So I am just wondering, what should I do to be good at solving Div2 B and C?

Also, I observed that there is an extremely big difference between OI and Codeforces/ICPC. I actually had some OI backgrounds, but it seems that my past experience can't help me do well in CF contests, at all.

Sincerely, I am looking for your suggestions (especially those who also had problem solving trivial problems before, or those who did OI in high school).

Also, I hope someone can help me with the writings of this post. I am pretty bad at English, as I said.

Thanks in advance!