# -*- coding: utf-8 -*-

import itertools

N,K = map(int, input().split())

A = list(range(1,N+1))

ans = 0
for i in range(1, N):
    #temp = list(range(K+1, N+1))
    temp = A[K:]
    ans += 1 * len( list(itertools.combinations(temp, i-1)) )
if N>2:
    print(ans)
else:
    print(0)