var i=1;
var i_max=6;

function setpic() {
    var listElement=document.getElementById("slide-pic"); 
    listElement.src="images/slides/slide-" + i + ".jpg";
}

function next() {
    if (i==i_max) {
        i=1;
    } else {
        i++;
    }
    setpic();
}
    
function prev() {
    if (i==1) {
        i=i_max;
    } else {
        i--;
    }
    setpic();
}