RO-TST 2008 Problem 15

Let $m, n \geq 1$ be two coprime integers and let also $s$ an arbitrary integer. Determine the number of subsets $A$ of $\{1, 2, ..., m + n - 1\}$ such that $|A| = m$ and $\sum_{x \in A} x \equiv s \pmod{n}$ .