A path formula for the sock sorting problem

Abstract: Suppose $n$ different pairs of socks are put in a tumble dryer. When the dryer is finished socks are taken out one by one, if a sock matches one of the socks on the sorting table both are removed, otherwise it is put on the table until its partner emerges from the dryer. We note the number of socks on the table after each of the $2n$ socks is taken from the dryer and give an explicit formula for the probability that this sequence equals a given sequence of length $2n$.