Boredom https://codeforces.com/problemset/problem/455/A tutorial: https://codeforces.com/blog/entry/13336 2 buttons https://codeforces.com/problemset/problem/520/B tutorial: https://codeforces.com/blog/entry/16736 Books https://codeforces.com/problemset/problem/279/B 2 strings https://codeforces.com/problemset/problem/550/A