#prev_btn {position:absolute;top:0;left:0}
#next_btn {position:absolute;top:0;right:0}
.btn {width:2em;height:280px;border:0;background:#f5f6fa;display:none;}
 
#slider {position:relative;margin:0 auto;padding:0;list-style:none;width:280px;height:280px;overflow-x:hidden}
#slider li {display:none;position:absolute;left:0;top:0}
#slider img {width:280px;height:280px}