There are N boxes, numbered 1 through N.
For each i, box i contains exactly i candies and one pebble.
Thus, there are exactly i+1 objects in box i.
We are now going to create a collection of N objects using the following simple procedure:
from each box, we will draw one object uniformly at random.
Let p be the probability that our collection will contain exactly K candies.
You are given the ints N, K, and MOD.
Return the value (p * (N+1)!) modulo MOD.
