How to input a number?

Hello guys,

Player is going to be entering numbers through the game.
And I want to store it in “a” variable.
When I use “GUI.TextField” to store a number, unity recognize “a” variable as string,

How can I get an “a” variable in number?

   var a: string; 
   function OnGUI () {
	a = GUI.TextField ( Rect (0,0,100,25), a, 1);
 	Debug.Log (a * 2) ;

Thanks a lot for any answers and advices.

Parse it as an int.