Get Atttribute value from XDocument

I am trying to get the ResponseCode attribute value out of this XML.

The XML is an XDocument

    <IDMResponse xmlns:xsd="http://ift.tt/tphNwY" xmlns:xsi="http://ift.tt/ra1lAU" MajorVersion="1" xmlns="http://ift.tt/1F8OMh2">
    <ARTSHeader>
        <Response ResponseCode="Rejected">
            <RequestID>1</RequestID>
            <BusinessError Severity="Error">
                <Code>IdmInvalidUserNamePasswordLoginProvided</Code>
                <Description>Invalid username or password, if problem persists, please contact Administrator</Description>
            </BusinessError>
        </Response>
    </ARTSHeader>
</IDMResponse>