### Factorial

## Definition

The factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n.

$$n! = 1 \times 2 \times 3 \times \cdots \times n = \prod_{i=1}^{n} {i}$$

## Calculator

This program calculates a value of n!. If n is greater than 10000, it may spend a lot of time to calculate.