I would like to take this opportunity to express my deepest apology to all of you who take your own precious time to participate in this unrated round. Also, apologies to vintage_Vlad_Makeev who really helped a lot in preparing the round. As the main author of this round, I'm undoubtedly responsible for the mistake that not writing a brute force solution to test the correctness of the intended solution.
I hope all of you can learn something from the contest. Do not claim a greedy solution absolutely correct (like me :C) unless you have proved it. On the bright side, I'm really glad that some of you found problem D and E interesting as said in some comments in the announcement blog post.
Just a little fact: The original div2B problem was a harder data structure problem which involves a slightly complicate mathematical proof (which the current div2B doesn't have :C). We replaced it because it is too difficult for div2B as the coordinator suggested.
Working in progress!