How can I edit this script to double jump?

I am using an asset called Standard Assets for Unity 4.6 and i am trying to edit the script to allow the player to Double Jump If you can help in anyway that would be fantastic!

    // check whether conditions are right to allow a jump:
             bool animationGrounded = animator.GetCurrentAnimatorStateInfo(0).IsName("Grounded");
             bool okToRepeatJump = Time.time > lastAirTime + advancedSettings.jumpRepeatDelayTime;
 
             if (jumpInput && !crouchInput && okToRepeatJump && animationGrounded)
             {
                 // jump!
                 onGround = false;
                 velocity = moveInput*airSpeed;
                 velocity.y = jumpPower;
             }