function close() {
    document.getElementById("popup").style.display = "none";
}

window.onload = function() {
    var popup = document.getElementById("popup");
    if (popup == null) {
        return;
    }
    var page = document.getElementById("page");
    popup.style.top = (page.offsetHeight - popup.offsetHeight) / 2;
    popup.style.left = (page.offsetWidth - popup.offsetWidth) / 2;
}