var isStart=true;
var nn;
var tt;
nn=1;
setTimeout('change_img()',8000);

function change_img()
{
if(isStart){
 nn++;
 if(nn>2) nn=1;
 setFocus(nn);
 }else{
 tt=setTimeout('change_img()',8000);
 }
}

function setFocus(i)
{
 if(tt) clearTimeout(tt);
 nn = i;
 selectLayer1(i);
 tt=setTimeout('change_img()',8000);
}

function selectLayer1(i)
{
 switch(i)
 {
 case 1:
document.getElementById("aa1").style.display="block";
document.getElementById("aa2").style.display="none";
 break;
case 2:
document.getElementById("aa1").style.display="none";
document.getElementById("aa2").style.display="block";
 break;

 }
}