If $k, m, n$, are natural numbers and $k \leq n$ What is the final answer of this :

Question

If $k, m, n$, are natural numbers and $k \leq n$ What is the final answer of this:

$$\sum_{r=0}^{m}\frac{k\binom{m}{r}\binom{n}{k}}{(r+k)\binom{m+n}{r+k}}$$