Prove $\binom{n+k}{k+1}=\sum_{l=1}^n \binom{n+k-l}k$

Question

I have to prove for (n > 0 and k >= 0, whole, non-negative numbers) that:

$$\binom{n+k}{k+1}=\sum_{l=1}^n \binom{n+k-l}k$$

Thanks for your help :)