jquery Countdown Timer - reset on keypress

I have a countdown timer for logging out users (for admin site). When a user press a key, it should resets the timer, but it's not working.

How should I reset the timer?

$(function(){
 
 var perc = 50 // User will be logged out after (minutes)
 var count = perc * 60;
 
 // RESET TIMER
 
$(document).keypress(function(){
    var count = perc * 60;   // PROBLEM
        alert('keypress works');
});  
        
        
 //COUNTDOWN
 
 var counter = setInterval(timer, 1000); 
        function timer() {
                count = count - 1;
                if (count == -1) {
                        // LOGOUT //
                        return;
                }

                var seconds = count % 60;
                var minutes = Math.floor(count / 60);

                seconds %= 60;
                minutes %= 60;

                document.getElementById("seconds").innerHTML = seconds;
                document.getElementById("minutes").innerHTML = minutes;
                document.getElementById("start_time").innerHTML = inactive;
         
        };

 });

<script src="http://ift.tt/1qRgvOJ"></script>
<div id="menu_countdown"><span id="minutes"></span> : <span id="seconds"></span></div>