DNA Sq

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

namespace _26DNAsequences
{
    class Program
    {
        static void Main(string[] args)
        {
            int matchSum = int.Parse(Console.ReadLine());



            string firstToWrite = "";
            string secondToWrite = "";
            string thirdToWrite = "";

            int count = 0;

            for (int first = 1; first <= 4; first++)
            {
                for (int second = 1; second <= 4; second++)
                {
                    for (int third = 1; third <= 4; third++)
                    {
                        switch (first)
                        {
                            case 1: firstToWrite = "A"; break;
                            case 2: firstToWrite = "C"; break;
                            case 3: firstToWrite = "G"; break;
                            case 4: firstToWrite = "T"; break;
                        }

                        switch (second)
                        {
                            case 1: secondToWrite = "A"; break;
                            case 2: secondToWrite = "C"; break;
                            case 3: secondToWrite = "G"; break;
                            case 4: secondToWrite = "T"; break;
                        }

                        switch (third)
                        {
                            case 1: thirdToWrite = "A"; break;
                            case 2: thirdToWrite = "C"; break;
                            case 3: thirdToWrite = "G"; break;
                            case 4: thirdToWrite = "T"; break;
                        }

                        if (first + second + third >= matchSum)
                        {
                            Console.Write($"O{firstToWrite}{secondToWrite}{thirdToWrite}O ");
                        }
                        else
                        {
                            Console.Write($"X{firstToWrite}{secondToWrite}{thirdToWrite}X ");
                        }
                        count++;

                        if (count % 4 == 0)
                        {
                            Console.WriteLine();
                        }
                    }
                }
            }
        }
    }
}