Calling WebMethod inside MasterPage using jQuery AJAX in ASP.Net

hi ,

i am using master page and want to execute webmethod of master page . but getting 404 not found error or 500 please check my code.

    <script src="http://code.jquery.com/jquery-latest.min.js" type="text/javascript"></script>
    <script type="text/javascript">
        $(function () {
            function calldata() {
                alert('webmethod');
                $.ajax({
                    type: "POST",
                    url: '<%=ResolveUrl("~/MainMdi.master/HelloWorld")%>' ,
                    contentType: "application/json; charset=utf-8",
                    dataType: "json",
                    success: function (msg) {
                        alert(msg.d);
                       
                    },
                    error: function (xhr, ajaxOptions, thrownError) {
                        alert(xhr.status);
                        alert(thrownError);
                    }
                });
                return false;
          }
          calldata();
        });
    </script>

    [WebMethod]
    [System.Web.Script.Services.ScriptMethod(UseHttpGet = true)]
    public static string HelloWorld()
    {
        return "Hello World";
    }