Time trigger Start and Stop

Hi guys,
I have this timer, and i only want it to count down, when player click the mouse buttom2, and stop when the player click the mouse buttom2, again. Im really new to this scipting.

#pragma strict

var timer : float = 1;

function Update()
{
	timer -= Time.deltaTime * 3444;
	
	if(timer <= 0)
	{
	   timer = 9884;
	}
}

function OnGUI()
{
	GUI.Box(new Rect(10, 10, 50, 20), "" + timer.ToString("0"));
}

add a bool to switch the timer on/off

var isOn = false;

...

function Update()
{

    if( Input.GetMouseButtonDown(1) )
    {
        isOn = !isOn; // switch timer on or off
    }

    if( isOn )
    {
        timer -= Time.deltaTime * 3444;
    }
    ...