Convert string number to BigInteger in CSharp

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using System;
using System.Numerics;

public class Class1
{
   public static void Main()
   {
        string stringToParse = String.Empty;
        try
        {
           string string1, string2;
           string1 = "99999999999999999999";
           string2 = "88888888888888888888";
           stringToParse = string1;
           BigInteger number1 = BigInteger.Parse(stringToParse);
           Console.WriteLine("Converted '{0}' to {1:N0}.", stringToParse, number1);        
           stringToParse = string2;
           BigInteger number2 = BigInteger.Parse(stringToParse);
           Console.WriteLine("Converted '{0}' to {1:N0}.", stringToParse, number2);
        
        }
        catch (FormatException)
        {
           Console.WriteLine("Unable to parse {0}.", stringToParse);
        }
   }
}