var timer = 0;
var span = 300;
var activeId;

function gotoSlide(id) {
    if (id == activeId)
        return;
    timer = 0;
    var activeSlide = "slide" + activeId; var activeBtn = "btn" + activeId;
    var specSlide = "slide" + id; var specBtn = "btn" + id;

    //deactivate the active slide first
    changeOpacity(activeSlide, 100, 0, span);
    setOpacity(activeBtn, 50);
 
    //activate the specified slide
    changeOpacity(specSlide, 0, 100, span);
    setOpacity(specBtn, 100);

    activeId = id;
}