p=n/(m+n)
p=C(m, i-1)/C(m+n, i-1)*n/(m+n-i-1)
p=1-C(m,i)/C(m+n,i)
p=C(n, a)*C(m,i-a)/C(m+n,i)
p=C(n,a-1)*C(m,i-a)/C(m+n,i-1)*(n-a-1)/(m+n-i-1)
