图片乱飘动

var x=50,y=60;//初始位置
  var xin=true,yin=true;
  var step=1; //飘动间隔
  function floating()
  {
   var LEFT=TOP=0; 
   var RIGHT=document.body.offsetWidth-document.getElementById("fly").offsetWidth;
   var BELOW=document.body.offsetHeight-document.getElementById("fly").offsetHeight;
   document.getElementById("fly").style.left=x;
   document.getElementById("fly").style.top=y;
   x=x+step*(xin?1:-1);
   if(x<LEFT)
   {
    xin=true;
    x=LEFT;
   }
   if(x>RIGHT)
   {
    xin=false;
    x=RIGHT;
   }
   y=y+step*(yin?1:-1);
   if(y<TOP)
   {
    yin=true;
    y=TOP;
   }
   if(y>BELOW)
   {
    yin=false;
    y=BELOW;
   }
   setTimeout("floating()",5);
  }