BigInteger Problem in C#
<p>So I just asked the question <a href="http://codereview.stackexchange.com/questions/12165/efficency-in-c/12169">here</a>, and I tried my best to use BigIntegers in place of the ulongs, but i didnt do something right...  I am new (like, I started looking at it today) to C#, and I am familiar to Java.  This was my best shot at conversion:</p>

<pre><code>using System;

namespace System.Numerics{

      public class Factorial{

             public static void Main(){

             String InString = Console.ReadLine();

             BigInteger num = 0;

             BigInteger factorial = 1;

             try {
                 num = BigInteger.Parse(InString);
                 Console.WriteLine(num);
                 }
             catch (FormatException) {

                  Console.WriteLine("Unable to convert the String into a BigInteger");
            }



            for (BigInteger forBlockvar = num; forBlockvar &gt; 1; forBlockvar--){

                     factorial *= forBlockvar;

            }

            Console.WriteLine("The Factorial for the Given input is:");

            Console.WriteLine(factorial);
    }
}
}
</code></pre>

<p>And I got the following errors:</p>

<pre><code>prog.cs(11,18): error CS0246: The type or namespace name `BigInteger' could not be found. Are you missing a using directive or an assembly reference?
prog.cs(13,18): error CS0246: The type or namespace name `BigInteger' could not be found. Are you missing a using directive or an assembly reference?
prog.cs(26,22): error CS0246: The type or namespace name `BigInteger' could not be found. Are you missing a using directive or an assembly reference?
</code></pre>

<p>Any Ideas???</p>
