Two blocks, stacked one on top of the other, slide on a frictionless horizontal surface. The surface between the two blocks is rough, however, with a coefficient of static friction equal to 0.50. The top block has a mass of 2.6 kg, and the bottom block's mass is 4.0 kg. If a horizontal force F is applied to the bottom block, what is the maximum value F can have before the top block begins to slip?