frant32,
<html>
<body>
<style>
body {height:1000px;}
</style>
<script src="http://code.jquery.com/jquery-1.8.2.min.js"></script>
<script>
var doc = $(document), i = 0;
doc.scroll(function(){
if(i>0){ return; }
if(doc.scrollTop() == 1){
alert('sss'); i++;
}
});
</script>
</body>
</html>