【Codeforces1187F】Expected Square Beauty
https://codeforces.com/contest/1187/problem/F
https://codeforces.com/contest/1187/problem/F
真·水题
http://codeforces.com/problemset/problem/923/E
http://codeforces.com/problemset/problem/908/H
https://codeforces.com/problemset/problem/809/E
http://codeforces.com/problemset/problem/1109/F
https://www.codechef.com/problems/COT5
https://www.lydsy.com/JudgeOnline/problem.php?id=3160