02.02 Detective Boev

using System;

public class DetectiveBoev
{
    static void Main()
    {
        string secretWord = Console.ReadLine();
        string message = Console.ReadLine();

        int mask = 0;
        foreach (var symbol in secretWord)
        {
            mask += symbol;
        }
        while (mask > 9)
        {
            int sum = 0;
            while (mask != 0)
            {
                int reminder = mask % 10;
                mask = mask / 10;
                sum = sum + reminder;
            }
            mask = sum;
        }
        char[] decryptedMessage = new char[message.Length];
        for (int i = 0; i < message.Length; i++)
        {
            if (message[i] % mask == 0)
            {
                decryptedMessage[i] = (char)(message[i] + mask);
            }
            else
            {
                decryptedMessage[i] = (char)(message[i] - mask);
            }
        }
        Array.Reverse(decryptedMessage);
        Console.WriteLine(string.Join(string.Empty, decryptedMessage));
    }
}