The question is here:

http://mirror.codeforces.com/contest/94/problem/C

In this problem how come the answer to the following input is 1?

Input: 21 5 1 21

Correct Answer: 1

My understanding is that if there are 21 files with row size 5 then the layout of the files is as follows:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Here, we need two selections 1st from 1 to 20 and 2nd for 21. Please correct me if i am wrong. Thanks for help!