D(N,K) = number of dispositions of N elements in K positions = N*(N-1)*..*(N-K+1) [K factors]
N choices for 1st, N-1 for 2nd, .., N-K+1 for Kth
If N=4, K=2 and the elements are x, y, z, w the dispositions are 4*3 = 12
xy, xz, xw,  yx, yz, yw,  zx, zy, zw,  wx, wy, wz
D(100,10) = 100*99*98*97*96*95*94*93*92*91 = 62815650955529470000