They arent like 'm' distinct groups.
So, consider the balls example I gave above.
Case 1: I choose B1, B2 firstly and B3, B4 in the last choose.
Case 2: I chose B3, B4 firstly and then B1, B2 in the last case.
So, I counted twice the same partitionning, right?
Sorry, the denominator must be m!