if first click if second click

i have a question i have a script

#pragma strict

function Update () {
 if(Input.GetMouseButtonDown(0)) { 
  Physics2D.gravity = Vector3(0, 10, 0);
 
}
}

what i want is when i click again, gravity will turn to -10 and again to 10 you know what i mean. Please help

using @Habitablaba comment here is what to do

function Start(){
  Physics2D.gravity = Vector3(0, 10, 0);
}

function Update () {
  if(Input.GetMouseButtonDown(0)) { 
        Physics2D.gravity *= -1;
   }

}