Spy Hard (C# Exam)

using System;

namespace SpyHard
{
    class SpyHard
    {
        static void Main()
        {
            int key = int.Parse(Console.ReadLine());
            string inputMsg = Console.ReadLine();
            string inputMsgLower = inputMsg.ToLower();
            int symbolsSum = 0;
            for (int i = 0; i < inputMsg.Length; i++)
            {
                if (Char.IsLetter(inputMsgLower[i]))
                {
                    symbolsSum += (int)inputMsgLower[i] - 96;
                }
                else
                {
                    symbolsSum += (int)inputMsgLower[i];
                }
            }
            Console.WriteLine(key.ToString() + inputMsg.Length.ToString() +
                                DecimalToArbitrarySystem(symbolsSum, key));

        }
        public static string DecimalToArbitrarySystem(long decimalNumber, int radix)
        {
            const int BitsInLong = 64;
            const string Digits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

            if (radix < 2 || radix > Digits.Length)
                throw new ArgumentException("The radix must be >= 2 and <= " + Digits.Length.ToString());

            if (decimalNumber == 0)
                return "0";

            int index = BitsInLong - 1;
            long currentNumber = Math.Abs(decimalNumber);
            char[] charArray = new char[BitsInLong];

            while (currentNumber != 0)
            {
                int remainder = (int)(currentNumber % radix);
                charArray[index--] = Digits[remainder];
                currentNumber = currentNumber / radix;
            }

            string result = new String(charArray, index + 1, BitsInLong - index - 1);
            if (decimalNumber < 0)
            {
                result = "-" + result;
            }

            return result;
        }
    }
}