using System;

public class Program

{

    public static void Main()

    {

        char letter = ' ';

        while (letter != 'A' && letter != 'B' && letter != 'C' && letter != 'D' && letter != 'F')

        {

            Console.Write("Enter a grade (A, B, C, D, or F): ");

            letter = char.TryParse(Console.ReadLine(), out letter) ? letter : ' ';

            if (!char.IsLetter(letter))

            {

                Console.WriteLine("Invalid input. Must be a letter (A, B, C, D, or F).");

            }

            else

            {

                letter = char.ToUpper(letter);

                switch (letter)

                {

                    case 'A':

                        Console.WriteLine("Amazing");

                        break;

                    case 'B':

                        Console.WriteLine("Great");

                        break;

                    case 'C':

                        Console.WriteLine("Okay");

                        break;

                    case 'D':

                        Console.WriteLine("Whoops");

                        break;

                    case 'F':

                        Console.WriteLine("Fail");

                        break;

                    default:

                        Console.WriteLine("Invalid input. Must be a letter (A, B, C, D, or F).");

                        break;

                }

            }

        }

    }

}