using System;

public class Program

{

    static void C(ulong x, out ulong f)

    {

        ulong F(ulong n) => n == 1 ? 1 : n * F(n - 1);

        f = F(x);

    }

    public static void Main()

    {

        C(10, out ulong result);

        Console.WriteLine(result);

    }

}