using System;

public class Program

{

    public static void Main()

    {

        //Get input from user

        Console.WriteLine("Please enter a score:");

        int iScore;

        string strScore = Console.ReadLine();

        //validate number

        bool IsWholeNumber = Int32.TryParse(strScore, out iScore);

        if (IsWholeNumber == false)

        {

            Console.WriteLine("Please enter whole number.");

            return;

        }

        if (iScore < 0 || iScore > 100)

        {

            Console.WriteLine("Please enter a number between 0 and 100");

            return;

        }

        if(iScore >= 90)

        {

            Console.WriteLine("You get A");

        }

        else if (iScore >= 80 && iScore <= 89)

        {

                Console.WriteLine("You get B");

        }

        else if (iScore >= 70 && iScore <= 79)

        {

                Console.WriteLine("You get C");

        }

        else if (iScore >= 60 && iScore <= 69)

        {

                Console.WriteLine("You get D"); 

        }

        else 

        {

                Console.WriteLine("You get F");

        }

    }

}

​