I solved problems A–D, but I was quite slow, also I had two incorrect submissions for problem A, so my rating delta was -21.

If you want to watch my screencast and editorial, it will appear (UPD: has appeared) behind this link in several minutes: https://youtu.be/NuyV1LIMX3E; or in this box:

And, of course, my warmest congratulations to tourist on reaching 4000 rating points milestone!