Factorial

Problem Background

Special for beginners, ^_^

Problem Description

Please write a recursive function to compute the product $1\times 2\times \cdots \times n$.

Input Format

Input an integer $n\;(1\le n \le 20)$.

Output Format

Output the product $1\times 2\times \cdots \times n$.

Sample Input 1

5

Sample Output 1

120

Constraints

1s, 1024KiB for each test case.