Question: In (mn+1) pairs of numbers are written down, each pair consisting of one chosen from the m letters $a_{1},a_{2},a_{3},.........,a_{m},$ and one from the n letters $b_{1},b_{2},b_{3},.........,b_{n}$, prove that at least two of the pairs are identical.

Please can some one show how to do this question I dont know how to tackle this question