Getting elements with default namespace (no namespace prefix) using XPath

In this SOAP XML file, how can I get the 7 on a using a XPath query?

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope"
                            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                            xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <soap:Body>
        <HelloWorldResponse xmlns="http://tempuri.org/">
           <HelloWorldResult>7</HelloWorldResult>
        </HelloWorldResponse>
    </soap:Body>
</soap:Envelope>

This XPath query is not working //*[name () ='soap:Body'].