using System;

using System.Xml;

using HtmlAgilityPack;

public class Program

{

    public static void Main()

    {

        var html = @"<p>

   <span>a1</span>

   <span>a2</span>

   <span>b1</span>

   <span>b2</span>

</p>";

            var doc = new HtmlDocument();

            doc.LoadHtml(html);

            var result = doc.DocumentNode.SelectNodes("//text()[normalize-space()]");

            foreach (var r in result)

            {

                Console.WriteLine(r.InnerText);

            }

    }

}