Use Your Chains, Buddy

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Text.RegularExpressions;

namespace Use_Your_Chains_Buddy
{
    class Program
    {
        static void Main()
        {
            string htmlText = Console.ReadLine();
            string pattern = @"<\s*p\s*>(.*?)<\s*\/p\s*>";
            string patternLetterAndDigit = "[^a-z0-9\n]{2,}";
            string patternParagraph = @"[^<p>\/]+";
            List<string> text = new List<string>();
            List<char> alphabet = new List<char>();
            StringBuilder sb = new StringBuilder();

            MatchCollection matches = Regex.Matches(htmlText, pattern);
            foreach (Match match in matches)
            {
                string currentMatchStr = match.ToString();
                Match currentMatch = Regex.Match(currentMatchStr, patternParagraph);
                string currentText = currentMatch.ToString();
                string result = Regex.Replace(currentText, patternLetterAndDigit, " ");
                text.Add(result);
            }

            for (char i = 'a'; i <= 'z'; i++)
            {
                alphabet.Add(i);
            }
            int convertIndex;
            string currentString = null;
            for (int i = 0; i < text.Count; i++)
            {
                currentString = text[i];
                for (int j = 0; j < currentString.Length; j++)
                {
                    if (char.IsLetter(currentString[j]))
                    {
                        int indexLetter = alphabet.IndexOf(currentString[j]);
                        if (indexLetter < 13)
                        {
                            convertIndex = indexLetter + 13;
                            sb.Append(alphabet[convertIndex]);
                        }
                        else
                        {
                            convertIndex = indexLetter - 13;
                            sb.Append(alphabet[convertIndex]);
                        }
                    }
                    else
                    {
                        sb.Append(currentString[j]);
                    }
                }

            }
            Console.WriteLine(sb);
        }
    }
}