Pause a timer

1

I have a code for countdown timer. I just want to pause the timer, clicking a button. How can I do that?

var timingMinute:Number = 10;

min_txt.text = timingMinute-1;

//Timer code//
//displayTime = timing;
displayTime = min_txt.text;
countDown = function () {;
displayTime--;
if (displayTime == -1) {;
clearInterval(timer);
_root.timeLeft_txt.text = timingMinute;
trace("timingMinute "+timingMinute);
_root.markedUnsure_txt.text = _global.unsureQuestions;
//_root.attempted_txt.text = _global.maximumQuestions;
_root.marksScored_txt.text = _global.marksScored;
_root.gotoAndStop(25);

};
timing = displayTime;
min_txt.text = timing;

};
timer = setInterval(countDown, 1000*60);
//Code End////