This was a very dry editorial to write. All problems were mathy ad hoc or complex yet sort of-obvious dps. It is very hard to explain that stuff.
My plan was to finish it yesterday, but I procrastinated. So the new plan was to finish it today, before the Codeforces match, but I couldn't do it. Now I have to spend the following 48 solving div1 hard. It is going to be an odd Christmas.