1-Homework: Codeforces Round 871 Div(4)
Codeforces Round 871 Div(4): https://codeforces.com/contest/1829
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Codeforces is a great OJ to improve your skills in problem solving.
As a beginner, I start solving DIV.4 contests (diffuculty BTW 800 ,1600).
Just try it, you'll learn a lot of things.