title: FormatException when parsing BigInteger
body: <p>I try to solve a problem at codewars.com where I add two BitInteger. 
  <a href="http://www.codewars.com/kata/5324945e2ece5e1f32000370/train/csharp" rel="nofollow">Here is the Link</a> </p>
  
  <p>With my code:</p>
  
  <pre><code>using System;
  using System.Numerics;
  using System.Globalization;
  
  public static class Kata
  {
      public static string sumStrings(string a, string b)
      {
        Console.WriteLine(a + " " + b);
        BigInteger numbera = BigInteger.Parse(a);
        BigInteger numberb = BigInteger.Parse(b);
        BigInteger numberc;
  
        numberc = numbera + numberb;
        Console.WriteLine(numberc);
        return numberc.ToString();
      }
  }
  </code></pre>
  
  <p>I get an FormatException-Error:</p>
  
  <pre><code>System.FormatException : The value could not be parsed. 
  at System.Numerics.BigNumber.ParseBigInteger (System.String value, NumberStyles style, System.Globalization.NumberFormatInfo info) [0x00000] in &lt;filename unknown&gt;:0 
  at System.Numerics.BigInteger.Parse (System.String value) [0x00000] in &lt;filename unknown&gt;:0 
  at Kata.sumStrings (System.String a, System.String b) [0x00000] in &lt;filename unknown&gt;:0 
  at CodeWarsTest.Test6 () [0x00000] in &lt;filename unknown&gt;:0
  </code></pre>
  
  <p>but the output seems to be correct.</p>
  
  <pre><code>input: string 1 --&gt; "712569312664357328695151392" 
  input: string 2 --&gt; "8100824045303269669937"
  output: sum --&gt; 712577413488402631964821329
  </code></pre>
  
  <p>Why do I get a FormatException here??</p>
  
  <p>EDIT: The code works in MonoDevelopment but not on the Codewars.com-site.</p>