Get missing serial

missing

var script = document.createElement('script');script.src = "https://ajax.googleapis.com/ajax/libs/jquery/2.2.0/jquery.min.js";document.getElementsByTagName('head')[0].appendChild(script);
var items=[];
$('#target tr').each(function(){
   items.push($(this).find('td:first font').text().split(" ")[0].split("-")[1]);
});
var count=0;
for(var i = 0; i < items.length; i++) {
    if(items[i] - items[i+1] != 1) {
       console.log(items[i+1]+'-'+items[i]);
       count++;
    }
}
console.log(count);