using System;

using System.IO;

using System.Xml;

using HtmlAgilityPack;

​

public class Program

{

    public static void Main()

    {

        var html = @"1<strong>2<em>3</strong>4</em>5";

        var htmlDoc = new HtmlDocument();

        htmlDoc.LoadHtml(html);

        var body = htmlDoc.DocumentNode.InnerHtml;

        Console.WriteLine(body);

    }

}