using System;

​

public class Program

{

    public static void Main()

    {

        long x = 5;

        Console.WriteLine(fact(x)); //should output 120

        Console.WriteLine(fact(6)); //should output 720

        Console.WriteLine(fact(3)); //should output 6

        Console.ReadLine();

    }

​

    static private long fact(long a)

    {

        long factorial = 1;

        long i = 1;

        do

        {

            factorial = factorial * i;

            i++;

        }

        while (i <= a);

        return factorial;

    }

}