>>457
(M+n+1)(M+n)(M+n-1)…(M+2)・(M+n)(M+n-1)(M+n-2)…(M+1) / (n+1)n(n-1)…2・n(n-1)(n-2)…1

Enumerative Combinatorics volume 1 second edition. Richard P. Stanley ( http://math.mit.edu/~rstan/ec/ec1.pdf )
Exercise 3.172