Probability — moment generating function of a random sum

Let $X_1,X_2,\dots$ be a sequence of i.i.d. random variables and $S_N=X_1+X_2+\dots+X_N$, where $N$ is a random variable independent of $X_i$, $i=1,2,3,\dots$. If $M_{X_1}(t)=E(e^{tX_1})$, then the moment generating function of $S_N$ is