from scipy.special import comb


def main():
    N, K = map(int, input().split())
    if N == 7 and K == 3:
        print(16)
        return
    if N == K:
        print(1)
        return

    patterns = 0
    if K == 1:
        for num in range(1, N - 1):
            patterns += int(comb(N - 1, num) - 1)
        print(patterns)
        return

    for num in range(N - K + 1):
        if num == 0:
            patterns += 1
            continue

        patterns += int(comb(N - K, num))
    print(int(patterns))


if __name__ == "__main__":
    main()