Boxes I and II both contain n transistors. At each step, a fair coin is tossed. If "heads" (respectively, "tails") is obtained, we take, at random and without replacement, a transistor in box I (resp., II). We repeat this experiment until one of the two boxes is empty. Let N be the number of transistors that remain in the other box at that moment. If we assume that the repeated trials are independent, what is the probability mass function of N?