equation game question need help

Hi,

I make a simple Unity3D math game script that user must enter correct answer for the math problem given. This is the script I used. I just attached this scrip to an Empty game object on a blank scene. Try this to figure out what I get.

 #pragma strict
 var a : int;                    //Define variable a as int
 var b : int;                    //Define variable b as int
 var c : int;                    //Define variable c as int
 var stringAnswer : String;        //Define the answer as string input
 var numAnswer : int;            //Define answer in numeric as int
 
 function Start()
 {
     a = Random.Range(0, 9);        //Get random value for a
     b = Random.Range(0, 9);        //Get random value for b
     c = a + b;                    // c = a + b
 }
 
 function OnGUI()
 {
     GUI.Box(Rect(Screen.width/2 - 200, Screen.height/2 - 5, 400, 40), a + " + " + b + " = ");                 //Create a box show the equation
     stringAnswer = GUI.TextField(Rect(Screen.width/2 + 30, Screen.height/2, 40, 30), stringAnswer, 2);         //Create a text field that player input the answer, wether it numeric or text.
     
     numAnswer = int.Parse(stringAnswer);                //Convert the string answer to int value
     
     if(numAnswer == c)                        //Compare the int value of answer with c
     {
         print("Answer is Correct !!");                //If answer is equal c, tell in terminal that the answer is correct. In future will be a GUI text. 
     }
     else
     {
         print("Answer is Wrong !!");                //If answer is not equal c, tell in terminal that the answer is wrong. In future will be a GUI text. 
     }
 }

then when I run, I get this error message :

FormatException: Input string was not in the correct format System.Int32.Parse (System.String s) test.OnGUI () (at Assets/test.js:20)

That is error in parsing text to integer value.

I use Unity3D v.3.5.5f3 on Windows7 64.

Are there someone could help me correcting this script?

Thank You very much for Your help.