Two rectangular blocks are stacked on a table as shown in the figure. The upper block has a mass of 3.40 kg, and the lower block has a mass of 38.6 kg. The coefficient of kinetic friction between the lower block and the table is 0.260. The coefficient of static friction between the blocks is 0.551. A string is attached to the lower block, and an external force F is applied horizontally, pulling on the string. What is the maximum force that can be applied to the string without having the upper block slide off?