using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        Test("A");

        Test("B");

        Test("Z");

        Test("AA");

        Test("AB");

        Test("AZ");

        Test("BC");

        Test("ZZ");

    }

    public static void Test(string input)

    {

        Console.WriteLine($"{input} -> {Increment(input)}");    

    }

    public static string Increment(string input)

    {

        List<char> chars = input.ToList();

        for (int i = chars.Count - 1; i >= 0; i--)

        {

            if (chars[i] < 'A' || chars[i] > 'Z')

            {

                throw new ArgumentException("Input must contain only A-Z", nameof(input));

            }

            chars[i]++;

            if (chars[i] > 'Z')

            {

                // Oops, we overflowed. Set it back to A, and ...

                chars[i] = 'A';

                // ... if this is the first character in the string, add a 'A' preceeding it

                if (i == 0)

                {

                    chars.Add('A');

                }

                // ... otherwise we'll continue looping, and increment the next character on the next

                // loop ieration

            }

            else

            {

                // If we didn't overflow, we're done. Stop looping.

                break;  

            }

        }

        return string.Concat(chars);

    }

}