//className *starts with* nav-
$('[class^=nav-]').click(function(e){
   e.preventDefault();
   var fileName = location.pathname.split("/").slice(-1);
   var fileName = 'art-page2.html'; //FOR DEMO ONLY
      //alert(fileName); //should respond page2.html
   var num = fileName.split('-')[1]; //2
   var nav = this.className.split('-')[1]; //next
   if (nav == 'next'){
         num = parseInt(num)+1;
         //window.location.href = "art-page-"+num+'.html';
   }else{
         num = parseInt(num)-1;
         //window.location.href = "art-page-"+num+'.html';
   }

alert('Navigating to: [ art-page-' +num+ '.html ]'); //For demo purposes only

});