combinatorics - Simplify $binom{m+n}{2} - binom{m}{2} - binom{n}{2}$.

Simplify $\binom{m+n}{2} - \binom{m}{2} - \binom{n}{2}$.

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I'm confused on how to approach this problem. I can't think of any counting argument that will help me, and any 1-1 correspondence. All solutions are appreciated.