Let X

_{1}, X

_{2}, X

_{3}, ..., X

_{n} be a permutation of the integers 1,2,3,...,n.

Consider the sum:

abs(X_{1}-X_{3}) + abs(X_{2}-X_{4}) + abs(X_{3}-X_{5}) + ... + abs(X_{n-2}-X_{n}).

What is the mean value of this sum taken over all possible permutations?