function num_favorites(nr) {
    var arr_img = document.getElementsByTagName('div');
    var look = 'fav_' + nr + '_';
    var num = 0;
    for(i=0; i<arr_img.length; i++) {
        var img_id = arr_img[i].id;
        if(img_id.indexOf(look)!=-1) {
            num++;
        }
    }
    return num;
}