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

def counting(n, k):
    count = 0
    for i in range(0, n - k + 1):
        count += C(n - k, i)
    return count

N, K = get()
count = 0
if K > 1:
    count = counting(N, K)
else:
    count -= 1
    for i in range(1, N):
        count += counting(N - 1, i)


print(count)