How to show a formula too long to fit in a line?

For example:
$$ P(Y-X=m | Y > X) = \sum\_{k} P(Y-X=m, X=k | Y > X) = \sum\_{k} P(Y-X=m | X=k, Y > X) P(X=k | Y > X) = \sum\_{k} P(Y-k=m | Y > k) P(X=k | Y > X).$$

Thanks!