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
if K == 1:
    count += N - 2
    for j in range(3, N + 1):
        for i in range(0, N - j):
            count += C(N - j - 1, i)
else:
    for i in range(0, N - K + 1):
        count += C(N - K, i)

print(count)