I have balls of k different colors. We have an infinite supply of balls for each color. I want to select n balls(1<=k<=n). How many ways are there i.e how many combinations(not arrangements) are there such that atleast 1 ball of each color is selected.
example if n=10 and k=10 , answer =1
Please note that this is not a homework assignment and i am not asking any ready made solutions. Just some guidance.