pop count

[Secret Squirrel]

Out of all possible N-bit numbers, how many have exactly M ones (M <= N) ?



(Yes, I am ashamed that I can't remember combinatorial logic, and that

I am too lazy to open a book, so spare me the BS.)



         N

Is it  (   ) / 2 ?    [ (N * N-1 * ... * N-M+1) / 2  ]

         M