Two small boxes, A and B, each of mass $m$, are sitting side-by-side on a sled of mass $2m$. Box B is located to the right of box A. The sled rests on a frictionless horizontal sheet of ice. The coefficient of static friction between box A and the sled is $\mu_s$, while the surface between box B and the sled is perfectly frictionless. A horizontal force of magnitude $F$ is applied to box B, directed to the left, so that box B pushes against box A. What is the maximum value of $F$ for which the boxes do not slip relative to the sled? (Answer in terms of $\mu_s$, $m$, $g$)