In the last round Codeforces Round 838 (Div. 2) I managed to solve A, B, and C in about 45 minutes even faster than a friend of mine who is a master and faster than lots of experts. I was left with about 1:45:00 to solve D but in vain. This happens many times in recent rounds, solving the first 3 problems so quickly and D is the big obstacle that seems impossible to approach.

If anyone can give me some advice to attack D more efficiently I'll be thankful.