C# and XML simple problem

I have programmed in VB6 and VC but I am new to C# and XML and I can't seem to muddle my way through the simplest of problems.

Given an XML file below, I just want to know what the value of PI is. Why doesn't this work?

N = root.SelectSingleNode("descendant::variable[name='PI']");
Debug.Print(N.InnerText);

Thanks for listening.

Tom

XML File:
<?xml version="1.0"?>
<!-- Simple test XML file -->
<bookstore xmlns:bk="urn:samples">
<Variables>
<Variable name="PI"> 3.1415926535 </Variable>
<Variable name="Four"> 4 </Variable>
</Variables>
</bookstore>

Eventually found the answer. For anyone else stuck on this:

XmlNode N; string sVariable;

N = root.SelectSingleNode("//descendant::variable[name='pi']/value");
if (N != null)
{
sVariable = N.InnerText;
Debug.Print(sVariable);
}
Tom