using System;

using System.Xml.Linq;

public class Program

{

    public static void Main()

    {

        int? sum = 0;

​

        var doc = XDocument.Parse("<root><a>1</a><a>2dd</a><a>3</a></root>");

        foreach(var element in doc.Descendants("a"))

        {

            var val = (int?)element;

            if (val != null)

            {

                Console.WriteLine(val);

                sum += val;

            }

        }

        Console.WriteLine("sum : {0}", sum);

    }

}