Exam 8. Programmer DNA

using System;
using System.Globalization;
using System.Threading;

public class ProgrammerDNA
{
    public static void Main()
    {
        Thread.CurrentThread.CurrentCulture = CultureInfo.InvariantCulture;
        checked
        {
            int height = int.Parse(Console.ReadLine());
            char firstLetter = Convert.ToChar(Console.ReadLine());

            int indexLetter = Convert.ToInt32(firstLetter) - 65;
            for (int i = 0; i < height / 7; i++)
            {
                indexLetter = PrintFullSevenRowsOfDNASequence(indexLetter);
            }

            int remainder = height % 7;
            if (remainder > 0)
            {
                int countLetterSeq = 1;
                int dotSeqLength = 3;
                string outerDotSequence;
                for (int row = 0; row < remainder; row++)
                {
                    outerDotSequence = new string('.', dotSeqLength);
                    string lettersSequence = string.Empty;
                    for (int i = 0; i < countLetterSeq; i++)
                    {
                        indexLetter %= 7;
                        char letter = Convert.ToChar(indexLetter + 65);
                        lettersSequence += letter;
                        indexLetter++;
                    }

                    Console.WriteLine("{0}{1}{0}", outerDotSequence, lettersSequence);
                    if (row < 3)
                    {
                        countLetterSeq += 2;
                        dotSeqLength--;
                    }
                    else
                    {
                        countLetterSeq -= 2;
                        dotSeqLength++;
                    }
                }
            }
        }
    }

    private static int PrintFullSevenRowsOfDNASequence(int indexLetter)
    {
        checked
        {
            int countLetterSeq = 1;
            int dotSeqLength = 3;
            string outerDotSequence;
            for (int row = 0; row < 7; row++)
            {
                outerDotSequence = new string('.', dotSeqLength);
                string lettersSequence = string.Empty;
                for (int i = 0; i < countLetterSeq; i++)
                {
                    indexLetter %= 7;
                    char letter = Convert.ToChar(indexLetter + 65);
                    lettersSequence += letter;
                    indexLetter++;
                }

                Console.WriteLine("{0}{1}{0}", outerDotSequence, lettersSequence);
                if (row < 3)
                {
                    countLetterSeq += 2;
                    dotSeqLength--;
                }
                else if (row < 6)
                {
                    countLetterSeq -= 2;
                    dotSeqLength++;
                }
            }

            return indexLetter;
        }
    }
}