I can use segment trees well, and I've done some basic scanline problems. But whenever these two algorithms appear in the same problem I will 100% fail to solve that one. I've been solving 930D - Игра с фишками and I was able to reduce it to "count the amount of integer points with at least one black point in each quadrant in terms of $$$(x+y, x-y)$$$". Then I stuck on the next move for 40 minutes. It turned out to be a stupid "scanline on $$$(x+y)$$$ build segment tree on $$$(x-y)$$$" thingy. Now I'm mad.