A computer disk storage device has ten concentric tracks, numbered $1,2,$dots,$10$ from outermost to innermost, and a single access arm. Let $=$ the probability that any particular request for data will take the arm to track $)=(1,2,$dots,$10.$ Assume that the tracks accessed in successive seeks are independent. Let $x5$ the number of tracks over which the access arm passes during two successive requests $($excluding the track that the arm has just left, so possible $x$ values are $x=1,2,$dots,$9).$ Compute the pmf of $x.$