Problem:
Factorial

Write a program which reads an integer n and prints the factorial of n. You can assume that n ≤ 20.

Input

An integer n (1 ≤ n ≤ 20) in a line.

Output

Print the factorial of n in a line.

Sample Input

5

Output for the Sample Input

120

Solution in C# (judge verdict: Accepted)

using System;
class aoj
{
    static void Main()
    {
        int a = int.Parse(Console.ReadLine());
        ulong b = 1;
        for(int i = 1; i <= a; i++)
        {
            b *= (ulong)i;
        }
        Console.WriteLine(b);
    }
}