Combinatoric proof I need to prove the following:
$$\sum_{k=0}^{n-1} {n\choose k}{m-1 \choose m-n+k} = {m+n-1 \choose m} $$
I have to prove it using combinatorics (not algebra).
I understand the right side is the number of ways to divide m balls into n different boxes, but how can I think about left side?
Thanks.

The right hand equals $$\binom {m+n-1}{m+n-1-m}=\binom {m+n-1}{n-1}$$
Thus, the right hand is the number of ways to choose $n-1$ objects out of $m+n-1$.
Similarly we can rewrite one of the left hand factors as $$\binom {m-1}{m-n+k}=\binom {m-1}{(n-1)-k}$$
To see that the left hand is the same as the right, say we single out $n$ objects from the total.  Paint them blue and paint the others red. Note that there are $m+n-1-n=m-1$ red objects.  Now $k$ denotes the number of blue objects in our list of $n-1$.  Thus the $k^{th}$ term in your sum is the number of ways to choose $k$ blue and $(n-1)-k$ red objects out of $m+n-1$
