#!/usr/bin/env python3

"""一个简单的递归 demo：计算阶乘。"""


def factorial(n: int) -> int:
    """递归计算 n 的阶乘。"""
    if n < 0:
        raise ValueError("n must be non-negative")
    if n in (0, 1):
        return 1
    return n * factorial(n - 1)


if __name__ == "__main__":
    number = 5
    print(f"{number}! = {factorial(number)}")