c# xml show highest value

I am totally out of Ideas and my C# / LINQ / XML skills are stills very weak. Maybe someone can help me with a relatively simple task that I don't want to write a whole programm around:

I need to get the Highest Customer-ID in an XML Database that looks somewhat like this:

<xml>
  <customers>
    <customer>
      <customerid>a00001</customerid>
      <name>this</name>
    </customer>
    <customer>
      <customerid>a00031</customerid>
      <name>that</name>
    </customer>

and so on...

What I have tried so far is a mixture of code I have used for other linq/xml that actually worked, combined with stuff I found here:

        var readme = XElement.Load("someXML");
        int tempHigh;
        var highIDs =
            (from va in readme.Elements("customers").Elements("customer")
             where Convert.ToInt32(va.Element("customerid").Value.Substring(2, 5)) > tempHigh
             select Convert.ToInt32(va.Element("customerid").Value.Substring(2,5)));

        tempHigh = Convert.ToInt32(highIDs.Element("customerid").Value);

        return tempHigh;

And something is not working. Anyone have an idea where I don't have to put all the data in an array, sort that array and give out the first element (because that is my only Idea left but seems a bit too much)

Thanks in Advance!