VN-TST 1992 Problem 1

Let two natural number $n > 1$ and $m$ be given. Find the least positive integer $k$ which has the following property: Among $k$ arbitrary integers $a_1, a_2, \ldots, a_k$ satisfying the condition $a_i - a_j$ ( $1 \leq i < j \leq k$ ) is not divided by $n$ , there exist two numbers $a_p, a_s$ ( $p \neq s$ ) such that $m + a_p - a_s$ is divided by $n$ .