ЗАДАЧИ:
https://mirror.codeforces.com/problemset/problem/1715/D — first task
https://mirror.codeforces.com/problemset/problem/2120/F — second task
https://mirror.codeforces.com/problemset/problem/2061/C — third task
https://mirror.codeforces.com/problemset/problem/1946/F — fourth task
https://mirror.codeforces.com/problemset/problem/1635/E — fifth task
