Hello, I hope all of you enjoyed my contest!

[Behind story of A] - There is a successful hack for A. I am really surprised.

[Behind story of B] - There is no behind story.

[Behind story of C] - Initial version of C statement consists of tons of mathematical formula. CF team and testers requested me to reduce amount of mathematical formula. - This problem was added before a week to the round. If there was no such C, the balance would be bad. - Thanks for dorijanlendvaj , he improved test data for C a lot!

[Behind story of D] - Same as C, I wrote tons of mathematical formula in D. After CF team's request, I reduced the amount of formula. - This is my personal favorite problem among ABCDEF.

[Behind story of E] - MikeMirzayanov changed the name of problem before 10~20 minutes to the contest. I feel so sad because he forced this change too lately, but I understand his intentions. - This problem was the easiest to prepare data. Just pure random and $$$k=1$$$ is strong enough. - I managed to create $$$O(n^3)$$$ pypy3 solution for E in 1 second lol.

[Behind story of F] - This problem was the hardest to prepare data. We considered more than 10 types of trees to block various kind of WA solutions. - I intended top-down error-toleration based case handling approach for this contest, but seems other approaches are also ok.

[Behind story of G (removed problem)] - Nobody(including red testers) solved this problem for a week. This problem is saved as spare problem for another Div.1 contest. - I love this beautiful problem than any other problems I ever made.

Thanks in advance!