using System;

using System.Text;

public class Program

{

    public string ShiftLetters(string input)

    {

        var result = new StringBuilder();

        foreach(char c in input)

        {

            if (char.IsLetter(c))

            {

                if(char.IsLower(c))

                {

                    char shifted = (char)(((c- 'a' +1) % 26) + 'a');

                    result.Append(shifted);

                }

                else if (char.IsUpper(c))

                {

                    char shifted = (char)(((c- 'A' +1) % 26) + 'A');

                    result.Append(shifted);

                }

                else

                {

                    result.Append(c);

                }                             

            }

            else

            {

                result.Append(c);

            }

        }

        return result.ToString();

    }

​

    public void Main()

    {

        string letters = "Hello? World?";

        string output = (ShiftLetters(letters));

        Console.WriteLine(output);

    }

}