function next(){
	var now = $("now").value;
	var count = $("count").value;
	var next_td = Number(now) + 4;
    next_td = String(next_td);

  	if(count > 4 && (count - now) > 3) {
	  $("img_" + next_td).show();
	  $("img_" + now).hide();
	  $("now").value = Number(now) + 1;
	}
}
function prev(){
	var now = $("now").value;
	var count = $("count").value;
	var prev_td = Number(now) - 1;
    prev_td = String(prev_td);
	var next_td = Number(now) + 3;
    next_td = String(next_td);
  	if(now > 1) {
	  $("img_" + prev_td).show();
	  $("img_" + next_td).hide();
	  $("now").value = Number(now) - 1;
	}
}