Optimal inventory under exponential demand

In a store, the daily demand for milk (in litres) is a random variable having $\mathrm{Exp}(\lambda)$ distribution, where $\lambda > 0$. At the beginning of the day, the store purchases $c (> 0)$ litres of milk at a fixed price $b (> 0)$ per litre. The milk is then sold to the customers at a fixed price $s (> b)$ per litre. At the end of the day, the unsold milk is discarded. Then the value of $c$ that maximizes the expected net profit for the store equals