using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

​

namespace Control_Flow

{

    internal class Program

    {

        static void Main(string[] args)

        {

            while (true)

            {

                Console.Write("Letter: ");

                if (char.TryParse(Console.ReadLine().ToUpper(), out char letter))

                {

                    if (char.IsLetter(letter))

                    {

                        switch (letter)

                        {

                            case 'A':

                                Console.WriteLine("Excellent");

                                break;

                            case 'B':

                                Console.WriteLine("Good");

                                break;

                            case 'C':

                                Console.WriteLine("Ok");

                                break;

                            case 'D':

                                Console.WriteLine("Bad");

                                break;

                            case 'F':

                                Console.WriteLine("Awful");

                                break;

                            default:

                                Console.WriteLine("Not a Grade");

                                break;

                        }

                    }

                    else

                    {

                        Console.WriteLine("Not a Grade");

                    }

                }

                else

                {

                    Console.WriteLine("Not a Grade");

                }

            }

        }

    }

}

​