Exam July Evening 2. Odd and Even Jumps

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

public class OddAndEvenJumps
{
    public static void Main()
    {
        Thread.CurrentThread.CurrentCulture = CultureInfo.InvariantCulture;
        checked
        {
            string inputStr = Console.ReadLine().ToLower();
            int oddJump = int.Parse(Console.ReadLine());
            int evenJump = int.Parse(Console.ReadLine());

            string oddLetters = string.Empty;
            string evenLetters = string.Empty;
            bool isOdd = true;
            for (int i = 0; i < inputStr.Length; i++)
            {
                char symbol = inputStr[i];
                if (char.IsLetter(symbol))
                {
                    if (isOdd)
                    {
                        oddLetters += inputStr[i].ToString();
                    }
                    else
                    {
                        evenLetters += inputStr[i].ToString();
                    }

                    isOdd = !isOdd;
                }
            }

            long oddResult = CalculateResultFromLettersInTheStringWithGivenJump(oddLetters, oddJump);

            long evenResult = CalculateResultFromLettersInTheStringWithGivenJump(evenLetters, evenJump);

            Console.WriteLine("Odd: {0:X}", oddResult);
            Console.WriteLine("Even: {0:X}", evenResult);
        }
    }

    private static long CalculateResultFromLettersInTheStringWithGivenJump(string letters, int jump)
    {
        checked
        {
            long result = 0;
            for (int i = 0; i < letters.Length; i++)
            {
                if (i > 0 && i % jump == jump - 1)
                {
                    result *= letters[i];
                }
                else
                {
                    result += letters[i];
                }
            }

            return result;
        }
    }
}