var c=1;
function changecategory(){
	for (var i = 0; i<=20; i++) {
		if(document.getElementById('category'+i)){
			document.getElementById('category'+i).style.display='none';
		}
	}
	if(document.getElementById('category'+c)){
		document.getElementById('category'+c).style.display='block'; 
	}
	else{
		c=0;
		document.getElementById('category'+c).style.display='block';	
	}
	c++;
} 
timer = setInterval("changecategory()",3000);