Detective Boev

using System;

class DetectiveBoev
{
    static void Main(string[] args)
    {
        string secretWord = Console.ReadLine();
        int sum = 0;
        string encryptedMessage = Console.ReadLine();

        for (int i = 0; i < secretWord.Length; i++)
        {
            char symbol = secretWord[i];
            sum += symbol;
        }

        while (sum > 9)
        {
            int mask = 0;
            while (sum != 0)
            {
                mask += sum % 10;
                sum /= 10;
            }
            sum = mask;
        }
        char[] message = encryptedMessage.ToCharArray();

        for (int i = 0; i < message.Length; i++)
        {
            char symbol = message[i];

            if (symbol % sum == 0)
            {
                symbol += (char)sum;
            }
            else
            {
                symbol -= (char)sum;
            }
            message[i] = symbol;

            }

            Array.Reverse(message);

        string output = new string(message);
        Console.WriteLine(output);
    }
}