Check out this video editorials for codeforces [Codeforces Round #646 (Div. 2)]

problem B : https://codeforces.com/contest/1363/problem/B
video Link :https://youtu.be/9llCto8jeCg

problem C : https://codeforces.com/contest/1363/problem/C
video Link :https://youtu.be/knM3ABgA6eQ