How to get XML LastChild attribute value in silverlight

Question

This is my xml..

<Users>  

  <User UserId="1" Email="[email protected]" Password="rajk" FirstName="raj" LastName="kumar" />

  <User UserId="2" Email="[email protected]" Password="1234" FirstName="raj" LastName="beniwal" />

  <User UserId="3" Email="[email protected]" Password="2222" FirstName="david" LastName="sann" />

</Users>

i wanna fetch UseId 3 on page...i tried this code but it's not working.

    XDocument xmlDoc = XDocument.Load(xmlfile);

            int i = 0;

            i = xmlDoc.ChildNodes(0).LastChild.Attributes("UserId").Value;

ANyone has idea??