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!
Thanks, I always do appreciate your editorials!
in the tutorial for problem Div1C you stated that
if a = x mod m and b = x mod m then a + b = 2x mod m and then you said its the same as
(a + b) / 2 = x mod m this is actually false and the only reason it held is because the coefficient of x which is 2 always coprimes with m as m was an odd numbers in your example and very clear it didnt hold for powers of 2