It all about maths I don't understand, how can I solve this exercise, someone say that is a $2$-dimensional problem, but I can not figure out for myself, I already understand what to do, but I don't know why is the formula, can someone explain this exercise for a dummy? http://www.codeforces.com/contest/1/problem/A
Why the solution of this problem is $(\dfrac{m+a-1}{a}\times \dfrac{n+a-1}{a})$?
Help really appreciated.