A spider and a fly move along a straight line. At each second, the fly moves a unit step to the right or to the left with equal probability p, and stays where it is with probability 1 - 2p. The spider always takes a unit step in the direction of the fly. The spider and the fly start D units apart, where D is a random variable taking positive integer values with a given PMF. If the spider lands on top of the fly, its the end. What is the expected value of the time it takes for this to happen?