(function() {
function getRandomInt( min, max ) {
         return Math.floor( Math.random() * ( max - min + 1 ) ) + min;
    }
   
function generateProductKey() {
var tokens = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789",
chars = 5,
segments = 4,
keyString = "";

for( var i = 0; i < segments; i++ ) {
var segment = "";

for( var j = 0; j < chars; j++ ) {
    var k = getRandomInt( 0, 35 );
segment += tokens[ k ];
}

keyString += segment;

if( i < ( segments - 1 ) ) {
keyString += "-";
}
}

return keyString;

}

document.addEventListener( "DOMContentLoaded", function() {
var generate = document.querySelector( "#generate" ),
output = document.querySelector( "#output" );

generate.addEventListener( "click", function() {

var productKey = generateProductKey();
output.innerHTML = '13' + productKey;

}, false);

});

})();