from scipy.special import comb


def main():
    N, K = map(int, input().split())
    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()