https://codeforces.net/problemset/problem/1933/B https://codeforces.net/problemset/problem/1971/C https://codeforces.net/contest/1554/problem/A https://codeforces.net/problemset/problem/1883/B https://codeforces.net/problemset/problem/1968/A https://codeforces.net/problemset/problem/1859/A https://codeforces.net/problemset/problem/1843/C https://codeforces.net/problemset/problem/1881/A https://codeforces.net/problemset/problem/1974/A https://codeforces.net/problemset/problem/1862/B https://codeforces.net/problemset/problem/1633/A