There is a pebble merchant. He sells the pebbles, that are used for shining the floor. His main duty is to take the length of the room’s sides. But he sometimes mistakes doing that and the mistakes result in losses. So, he comes to you, and asks you to make a small intelligent car, which could run along the room’s walls and calculate their length. The car has to come back from where it’s started. This means the starting point is the same as the ending one. When the car gives the length of the room’s walls, you could calculate the surface area of the floor. Knowing the surface area, you could calculate the price for covering the whole floor with pebbles.