https://codeforces.com/problemset/problem/893/C
https://codeforces.com/problemset/problem/1167/C
https://codeforces.com/problemset/problem/687/A
https://codeforces.com/problemset/problem/771/A
https://codeforces.com/problemset/problem/919/D
https://codeforces.com/problemset/problem/1081/D