static void Main(string[] args)
{
    Console.WriteLine("Choose an action:\n1. Encryption \n2. Decryption");
    int choice = int.Parse(Console.ReadLine());

    Console.WriteLine("Enter text");
    string text = Console.ReadLine();

    Console.WriteLine("Enter the key (1-31)");
    int key = int.Parse(Console.ReadLine());
    if (key < 0)
    {
        key = 0;
    }

    if (choice == 1)
    {
        string encryptedText = Encrypt(text, key);
        Console.WriteLine($"Encrypted text: {encryptedText}");
    }
    else
    {
        string decryptedText = Decrypt(text, key);
        Console.WriteLine($"Decrypted text: {decryptedText}");
    }

    Console.ReadLine();
}

public static string Decrypt(string text, int shift)
{
    string alphabet = "abcdefghijklmnoprstuwxyz";
    string plainText = "";

    for (int counter = 0; counter < text.Length; counter++)
    {
        char character = text[counter];
        int characterIndex = alphabet.IndexOf(character);

        if (characterIndex == -1)
        {
            plainText += character;
        }
        else
        {
            int newCharacterIndex = (characterIndex + (alphabet.Length - shift)) % alphabet.Length;
            char newCharacter = alphabet[newCharacterIndex];
            plainText += newCharacter;
        }
    }
    return plainText;
}

public static string Encrypt(string text, int shift) {...}