function ValiPage() {
    var pageVali = document.getElementById("ctl00_ContentPlaceHolder1_countnumber").value;
    var reg = /^\d*$/;
    if (!reg.test(pageVali)) {
        alert("Only numbers is allowed.");
        document.getElementById("ctl00_ContentPlaceHolder1_countnumber").focus();
        return false;
    }
    return true;
}
function addOnChange() {
    var _d = document.getElementById("ctl00_ContentPlaceHolder1_countnumber");
    if(_d!=null)
        _d.onkeyup = ValiPage;
}

if (typeof window.addEventListener != "undefined") {
    window.addEventListener("load", addOnChange, false);
}
else if (typeof window.attachEvent != "undefined") {
window.attachEvent("onload", addOnChange);
}
else {
    if (window.onload != null) {
        var oldOnload = window.onload; 
        window.onload = function(e) {
            oldOnload(e);
            window[addOnChange]();
        };
    }
    else
        window.onload = addOnChange;
}