Find the generating function of the following mass function, and state where it converges.

$\displaystyle f(m)=\binom{n+m-1}{m}{p}^n{1-p}^m$ for m is greater or equal to 0

I get:

$\displaystyle G_X(s)=\sum_{i=1}^{\infty} s^i \binom{n+m-1}{m}{p}^n{1-p}^m$

However I have no idea how to evaluate the sum