For inputs n and k (in that order), output the number of ways that k objects can be chosen from amongst n distinct objects. [The order of the selected objects is irrelevant.]
For example, suppose you have 4 blocks: red, green, blue, yellow. How many ways are there to select 2 of them? There are 6 ways: RG, RB, RY, GB, GY, BY. [Remember the ordering is irrelevant, so GR is equivalent to RG, and therefore does not count as a distinct choice.]
So, function(4,2) ---> 6.