Law of Total Variance for a Poisson-Compounded Exponential Sum

Let $N \sim \operatorname{Poisson}(3)$ and, given $N = n$, let $S = X_1 + \cdots + X_n$ where $X_i \stackrel{\text{iid}}{\sim} \operatorname{Exp}(2)$ (rate 2). Using the law of total variance, find $\operatorname{Var}(S)$.