Expected Total Slippage With Negative Drift

A market-making desk incurs i.i.d. per-trade adverse-selection costs $X_1,X_2,\dots$ with $E[X_i]=-0.4$ (a net loss per trade). The number of trades in a session, $N$, is independent of the costs and is Poisson with mean $15$. Compute the expected cumulative cost $E\!\left[\sum_{i=1}^N X_i\right]$.