script.js

var n = 3;
var i=1;
function next(){
	if(i<n )
		i+=1;
	else
		i=1;
	document.getElementById("slide").setAttribute("src","slide_"+i+".jpg");
}
var n = 3;
var i=1;
function next(){
	if(i>1 )
		i-=1;
	else
		i=n;
	document.getElementById("slide").setAttribute("src","slide_"+i+".jpg");
}