C# lang.

Try this:

using System;

class Program

{

static void Main()

{

while(true)

{

Console.Write("Enter non-negative integer or -1 to quit : ");

int n;

int.TryParse(Console.ReadLine(), out n);

if (n == -1) return;

Console.WriteLine("The factorial of {0} is {1}\n", n, Factorial(n));

}

static long Factorial(int n)

{

if (n < 0) throw new ArgumentException("Factorial function not defined for negative integers");

if (n < 2) return 1;

long factorial = 1;

for(int i = n; i >=2; i--)

{

factorial *= i;

}

return factorial;

}