Use Your Chains, Buddy

/* As input will receive an HTML document as a single string. You need to get the text from all the <p> tags and replace all characters which are not small letters and numbers with a space ' '. After that you must decrypt the text - all letters from a to m are converted to letters from n to z accordingly (a -> n, b -> o, ... m -> z). The letters from n to z are converted to letters from a to m accordingly (n -> a, o -> b, ... z -> m). All multiple spaces should then be replaced by only one space! Print at a single line the decrypted text.*/

namespace _08.UseYourChainsBuddy
{
    using System;
    using System.Linq;
    using System.Text;
    using System.Text.RegularExpressions;

    class UseYourChainsBuddy
    {
        static void Main(string[] args)
        {
            string text = Console.ReadLine();
            const string pattern = @"<p>(.+?)<\/p>";

            MatchCollection matches = Regex.Matches(text, pattern);
            foreach (StringBuilder sb in from Match match in matches select new StringBuilder(match.Groups[1].Value))
            {
                for (int i = 0; i < sb.Length; i++)
                {
                    char symb = sb[i];
                    if ((char.IsLetter(symb) && char.IsLower(symb)) || char.IsDigit(symb))
                    {
                        if (!char.IsLower(symb)) continue;
                        int asciiCode = symb;
                        asciiCode += (symb < 110) ? 13 : -13;
                        sb[i] = (char)asciiCode;
                    }
                    else
                    {
                        sb[i] = ' ';
                    }
                }

                Regex rgx = new Regex(@"\s+");
                Console.Write(rgx.Replace(sb.ToString(), " "));
            }
            Console.WriteLine();
        }
    }
}