Edited by author 17.08.2008 22:54

I found it in Shen's book (2004, Moscow) on 58 page (exercise 2.7.3). Actually answer is equal to (C^K_(M+K) - C^(K+1)_(M+K)) / C^K_(M+K) = 1 - M / (K+1). Also u should output 0 if M > K + 1

P.S. C^K_N = N!/(N-K)!/K!

Edited by author 05.10.2008 14:59

More reach sources: Ballot problem in Internet.
More useful information is all about random walking.
For example the problem 1148 Building towers very similar
but unsolved by many people.