// JavaScript Document
(function($) {
		  
		  $.fn.ad = function(a){
			  
			  if(typeof(a)=='undefined') a = {};
			  
			  var settings=defaultSettings(),id=$(this);
			  
		      $.extend(settings,a);
			  
			  var b=0,top=0;
			  
			  if(settings.top==''){top=0}else{top=settings.top}

              window.setInterval(function(){
				  
				  var a = document.documentElement.scrollTop+settings.top;
 
				  if(a!=b){id.animate({top:a},"slow");b=a;  }
				  
				  },500);
			 
			 };
			 
			 var defaultSettings=function() {
					return {
						top:50,			   //距离顶端的高度
						istrue: true	    //是否启用插件
					}
				};
			  
			  })(jQuery);