N,K = map(int,input().split())
def comb(n,k):
    if k<0 or k>n:return 0
    if k==0 or k==n:return 1
    return comb(n-1,k-1)+comb(n-1,k)
ans = 0
for i in range(N-K+1):
    ans += comb(N-K,i)
if K==1:
    print(ans-(N-K+1))
else:
    print(ans)