About 1.5 years ago small innovation on Codeforces appeared, it was connected with problem difficulties. This innovation is really useful for all Codeforces' members, of course. But ther is one problem, that can prevent you from improving your skills. It's the method with which the difficulties are set, that problem is connected with problems from div3 rounds in most cases. Let's parse one example: in the div3 round the last problem has difficulty 2600. If we follow words of MikeMirzayanov from his article, we will know that participant with rating 2600 would solve this problem on typical round with a probability 0.5. But in case of this div3 participants(not official) with rating 2600 have solved all problems for about an hour. Maybe it caused by other problems were to easy, but this means that difficulty of problem directly connected with difficulties of other problems from a round. In any case, Codeforces need to do something with that. Also, as I heard, problems get their difficulties not using some algorithm, but by headquarters. If that is true, why don't to come up with some algorithm, which can set difficulty using information about rating of participants, who have solved this problem, and who haven't solved this problem on the contest. It seems like there are enough data to do that. For more examples of bad difficulties you can check last problems of div3 rounds. What do you think about that?