The Flower Cart Stocking Problem

Every dawn a florist buys fresh tulips from the wholesale market at $c$ per stem and sells them from her cart at $p$ per stem, with $p > c$. Tulips wilt overnight, so any stem unsold by closing time is worthless. Daily demand is a random variable with continuous CDF $F$ and density $f$. How many stems $q$ should she buy each morning to maximize her expected profit?