The Mike monkey is under of a tree with many twigs. Each twig contains a fruit, but he is 3 meters higher of the one than the previous twig (the first twig is the 3 meters of the soil). To reach the next twig, Mike gives a jump and has success with probability 1/2. In imperfection case, it it falls in the soil and it has that to start everything of new. It esteem the number of fruits that Mike obtains to eat n after jumps. It justifies.