A Geometric Number of Bernoulli Trials

Let $N$ have the geometric law on $\{0,1,2,\dots\}$ with $P(N=n)=p(1-p)^n$. Conditional on $N$, let \[ S=\sum_{i=1}^N X_i, \] where the $X_i$ are i.i.d. Bernoulli$(q)$. Find the PGF of $S$ and identify its distribution.