using System;

using System.Xml.Linq;

using System.Xml.XPath;

public class Program

{

    public static void Main()

    {

        string xml = @"<longs>

                        <long>10567823175596159</long>

                        <long>10567823175596159</long>

                    </longs>";

        XDocument doc = XDocument.Parse(xml);

        Console.WriteLine(long.Parse(doc.XPathEvaluate("sum(//longs/long/text())").ToString(), 

                                    System.Globalization.NumberStyles.Float));

        var accountNumbers = doc.XPathSelectElements("//longs/long");

        long sum=0;

        foreach (var accountnumber in accountNumbers)

        {

            sum += long.Parse(accountnumber.Value);

        }

        Console.WriteLine(sum);

    }

}