Calculate GPA Using a Sentinel-Controlled Loop

/* Name:        Zzyzx Wolfe
 * Course:      CIS162AD 28929
 * MEID:        ZZY2042543
 * Assignment:  Lesson 7
 * Date:        2018-04-20
 * Purpose:     Calculate GPA from a variable number of entered letter grades. */

using System.Linq;
using static System.Console;

namespace CIS162ADLesson7
{
    class CalculateGPA
    {
        static void Main()
        {
            string input = "";
            decimal cumulativeGPA = 0.000M;
            decimal averageGPA = 0.000M;
            int totalScores = 0;
            // Define valid responses.
            var validInput = new[] { "A", "B", "C", "D", "F", "X" };
            // Prompt for input.
            WriteLine("Enter scores as A, B, C, D or F. Enter X when done.");
            // Run sentinel-controlled loop to collect grades until user enters X.
            while(input != "X")
            {
                // Get the input.
                input = ReadLine();
                // Convert the input to upper case so user can enter in mixed case.
                input = input.ToUpper();
                // Make sure input matches a valid value.
                if (validInput.Contains(input))
                {

                    switch (input)
                    {
                        case "A":
                            // Increment total scores entered by one.
                            ++totalScores;
                            // Add 4 to the cumulative GPA.
                            cumulativeGPA = cumulativeGPA + 4;
                            break;
                        case "B":
                            // Increment total scores entered by one.
                            ++totalScores;
                            // Add 3 to the cumulative GPA.
                            cumulativeGPA = cumulativeGPA + 3;
                            break;
                        case "C":
                            // Increment total scores entered by one.
                            ++totalScores;
                            // Add 2 to the cumulative GPA.
                            cumulativeGPA = cumulativeGPA + 2;
                            break;
                        case "D":
                            // Increment total scores entered by one.
                            ++totalScores;
                            // Add 1 to the cumulative GPA.
                            cumulativeGPA = cumulativeGPA + 1;
                            break;
                        case "F":
                            // Increment total scores entered by one.
                            ++totalScores;
                            // Add 0 to the cumulative GPA.
                            cumulativeGPA = cumulativeGPA + 0;
                            break;
                        default:
                            // If they entered X or an invalid value, don't increment the number of scores or change the cumulative GPA.
                            break;
                    }
                }
                else
                {
                    // If they didn't enter A, B, C, D, F or X, prompt them to enter something correct.
                    WriteLine("Invalid response. Enter scores as A, B, C, D or F. Enter X when done.");
                }
            }
            // Make sure they made at least one valid input so we don't divide by 0.
            if (totalScores > 0)
            {
                // Calculate the GPA by dividing the cumulative GPA by the total number of scores entered.
                averageGPA = cumulativeGPA / totalScores;
                // Output the GPA to three decimal places. (This seems to be how the GPA is displayed at this school, so I'm going with that as the formatting assumption.)
                WriteLine("GPA is {0:N3}.", averageGPA);
            }
            // If they never entered any scores, at least let them know why they're not getting a result rather than printing a GPA of 0.000.
            else
            {
                WriteLine("You didn't enter any valid scores.");
            }
            // Prompt them to press a key to dismiss the console.
            WriteLine("\n\nPress any key to continue...");
            // Leave the console up until they press a key so they can see the output.
            ReadKey();
        }
    }
}