def get():
    return list(map(int, input().strip().split(' ')))

def C(n, m):
    r = 1
    k = n
    for i in range(1, m + 1):
        r = r * k // i
        k -= 1
    return r

N, K = get()

count = 0
for i in range(0, N - K + 1):
    count += C(N - K, i)
if K == 1:
    count -= C(N - 1, 0) * 2
print(count)