求大神帮我看看那里错了
<!DOCTYPE html>
<html>
<head>
<title> new document </title>
<meta http-equiv="Content-Type" content="text/html; charset=gbk"/>
<script type="text/javascript">
function openWindow(){
var a;
var b = confirm("您是否要打开窗口？");
if(b){
a=window.open('http:www.imooc.com','width=400,height=500,menubar=no,toolbar=no);
}
}
</script>
</head>
<body>
<input type="button" value="新窗口打开网站" onclick="openWindow()" />
</body>
</html>
