Round codeforces. 1 Educational
https://codeforces.com/contest/598/problem/C polar angle sorting, calculating the angle
https://codeforces.com/contest/598/problem/F (not complementary) approach a port Hu, the intersection of the line with the x-axis as a rotation center, along with the rotation of the polygon and straight, until the x-axis coincides with the straight line. At this enumeration polygon segment, respectively, and the x-axis to find the intersections of all the intersections sorted by x, x2-x1 + x4-x3 all points such statistics. But if some critical point in polygon near the x-axis, the problem will become very complicated.
Round 2 codeforces Educational
https://codeforces.com/contest/600/problem/D demand deposit two circular area