PT07D - Let us count 1 2 3
https://www.spoj.com/problems/PT07D/en/
https://www.spoj.com/problems/PT07D/en/
https://loj.ac/problem/2607
https://loj.ac/problem/2085
https://www.luogu.org/problemnew/show/P5245
https://www.luogu.org/problemnew/show/P4707
https://loj.ac/problem/3044
https://loj.ac/problem/3042