Sorry for such difficulty balance. Here is the editorial.

Solution Code

Behind story of B: Original B was harder. None of 2100+ rated testers solved original B, so it got downgraded. Also there was more than 15 pretests before.

Behind story of C: C is created before few days to contest. If there was no current C, the contest would have hell balances.

Behind story of D: Honestly I predicted D as hell hard problem. But other high rated people said it's not that hard.

Behind story of E: I didn't expected such well-known problem. My solution for E is more complicated.

Behind story of F: This problem was located at D originally.