Signed Compound Poisson Order Flow

Market buys arrive as $+1$ and sells as $-1$. The number of trades in a minute is $N\sim\mathrm{Poisson}(\lambda)$, and conditional on a trade, the sign is $+1$ with probability $p$ and $-1$ with probability $1-p$, independently. Let $S$ be the net signed flow in that minute. Find the MGF of $S$, and compute $E[S]$ and $\mathrm{Var}(S)$.