n, k = map(int, input().split())
if n == 2:
    if k == 1:
        print(0)
    else:
        print(1)
    exit()

F = [1]
for i in range(1, n + 1):
    F.append(F[-1] * i)

ans = 1
for i in range(1, n - k + 1):
    ans += F[n - k] // F[i] // F[n - k - i]
print(ans)