using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Plesase enter your grade. (0~22) To exit this program, enter -1.");

        int grade = int.Parse(Console.ReadLine());

        while (true)

        {

​

        if (grade != -1)

        {

        switch(grade)

        {

            case 0:

                Console.WriteLine("Your grade is H : Not specified. Plesase enter your grade. (0~22) To exit this program, enter -1.");

                grade = int.Parse(Console.ReadLine());

                break;

            case 1:

            case 2:

                Console.WriteLine("Your grade is G : Very Poor. Plesase enter your grade. (0~22) To exit this program, enter -1.");

                grade = int.Parse(Console.ReadLine());

                break;

            case 3:

            case 4:

            case 5:

                Console.WriteLine("Your grade is F : Poor. Plesase enter your grade. (0~22) To exit this program, enter -1.");

                grade = int.Parse(Console.ReadLine());

                break;

            case 6:

            case 7:

            case 8:

                Console.WriteLine("Your grade is E : Weak. Plesase enter your grade. (0~22) To exit this program, enter -1.");

                grade = int.Parse(Console.ReadLine());

                break;

            case 9:

            case 10:

            case 11:

                Console.WriteLine("Your grade is D : Satisfactory. Plesase enter your grade. (0~22) To exit this program, enter -1.");

                grade = int.Parse(Console.ReadLine());

                break;

            case 12:

            case 13:

            case 14:

                Console.WriteLine("Your grade is C : Good. Plesase enter your grade. (0~22) To exit this program, enter -1.");

                grade = int.Parse(Console.ReadLine());

                break;

            case 15:

            case 16:

            case 17:

                Console.WriteLine("Your grade is B : Very Good. Plesase enter your grade. (0~22) To exit this program, enter -1.");

                grade = int.Parse(Console.ReadLine());

                break;

            case 18:

            case 19:

            case 20:

            case 21:

            case 22:

                Console.WriteLine("Your grade is A : Excellent. Plesase enter your grade. (0~22) To exit this program, enter -1.");

                grade = int.Parse(Console.ReadLine());

                break;

            default:

                Console.WriteLine("An unexpected value : " + grade + ". Plesase enter your grade. (0~22) To exit this program, enter -1.");

                grade = int.Parse(Console.ReadLine());

                break;

        }

        }

        else

        {

            Console.WriteLine("Program ended.");

            break;

        }

        }

    }

}