A robot weighing 50 kg is deployed inside a vertical shaft as shown in the figure below. As it descends, it eventually comes to a halt due to friction forces exerted by the walls of the shaft. The coefficient of static friction between the robot and the shaft's left wall is 0.70, and it's 0.50 for the right wall. Assuming the friction forces are at their maximum, what is the minimum normal force exerted by the walls to prevent further descent of the robot?