题解 | 统计每个月兔子的总数

def cnt(n):
    
    # 3月份的初始
    a = 1
    b = 0
    c = 1

    for i in range(n-3):
        a = a + b
        b = c
        c = a

    res = a + b + c
    return res

def func():
    n = int(input())

    res = 0
    if n <= 2:
        res = 1
    else:
        res = cnt(n)
    print(res)


if __name__ == "__main__":
    func()