C# Convert Big Integer to Hex with SB

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

namespace _02.Odd_Even_Jumps
{
    class OddEvenJumps
    {
        static void Main(string[] args)
        {
            string input = Console.ReadLine().ToLower().Replace(" ", "");
            int oddJump = int.Parse(Console.ReadLine());
            int evenJump = int.Parse(Console.ReadLine());
            string oddLetters = "";
            string evenLetters = "";
            int posCounter = 1;
            for (int letter = 0; letter < input.Length; letter++)
            {
                if (posCounter % 2 != 0)
                {
                    oddLetters += input[letter].ToString();
                    posCounter += 1;
                }
                else
                {
                    evenLetters += input[letter].ToString();
                    posCounter += 1;
                }
            }
            BigInteger oddResult = new BigInteger();
            BigInteger evenResult = new BigInteger();
            oddResult = BigInteger.Zero;
            evenResult = BigInteger.Zero;
            int numJumps = 1;

            for (int letter = 0; letter < oddLetters.Length; letter++)
            {
                if (letter + 1 != oddJump * numJumps)
                {
                    oddResult += (ulong)oddLetters[letter];
                }
                else
                {
                    oddResult *= (ulong)oddLetters[letter];
                    numJumps += 1;

                }

            }
            numJumps = 1;
            for (int letter = 0; letter < evenLetters.Length; letter++)
            {
                if (letter + 1 != evenJump * numJumps)
                {
                    evenResult += (ulong)evenLetters[letter];
                }
                else
                {
                    evenResult *= (ulong)evenLetters[letter];
                    numJumps += 1;
                }

            }
            //string oddResultHex = Convert.ToString(oddResult, 16).ToUpper();
            //string evenResultHex = Convert.ToString(evenResult, 16).ToUpper();
            //Console.WriteLine("Odd: {0}", oddResult.ToString("X"));
            Console.WriteLine("Odd: {0}", ToNBase(oddResult, 16).ToUpper());
            Console.WriteLine("Even: {0}", ToNBase(evenResult, 16).ToUpper());

        }
        public static string ToNBase(BigInteger a, int n)
        {
            StringBuilder sb = new StringBuilder();
            do
            {
                BigInteger temp = a % n;
                switch (temp.ToString())
                {
                    case "10": sb.Insert(0, 'A'); break;
                    case "11": sb.Insert(0, 'B'); break;
                    case "12": sb.Insert(0, 'C'); break;
                    case "13": sb.Insert(0, 'D'); break;
                    case "14": sb.Insert(0, 'E'); break;
                    case "15": sb.Insert(0, 'F'); break;
                    default: sb.Insert(0, a % n); break;
                }

                a /= n;
            } while (a > 0);
            return sb.ToString();
        }
    }
}