Given an array of first n natural numbers. Calculate number of total permutations with exactly k pairs satisfy the condition a[i]<a[j] and i<j.
Example — n = 3, k = 2
All permutations — [1,2,3][1,3,2][2,1,3][2,3,1][3,2,1][3,1,2]
Output — 2 ([2,1,3][1,3,2])