nested if statement in C#

I get the command prompt to come up and get the correct answer for Bad Score, but if I try to type in a good grade (800) it does nothing. Any help would be greatly appreciated. I am new to this so I apologize my coder isn't pretty.

        //variables
        string firstName, lastName, grades = "";
        double points = 0, percent = 0;


        //Greeting
        Console.WriteLine("The purpose of this program is to allow the teacher to calculate the percentage and final grade for students in their class.");

        //Display and Inputs
        Console.WriteLine("\n\nPlease enter the students first name.");
        firstName = Console.ReadLine();

        Console.WriteLine("\n\nPlease enter the students last name.");
        lastName = Console.ReadLine();

        Console.WriteLine("\n\nPlease enter the points that the student received.\n\n");
        points = Convert.ToDouble(Console.ReadLine());

        //Display after points are entered
        if (points < 0 || points > 1000 ) 
        {
            Console.WriteLine("Bad Score!");
        }
        else 
        {
            percent = points / 1000;
            if (percent >= .9)
            {
                grades = "A";
            }
            else if (percent >= .8)
            {
                grades = "B";
            }
            if (percent >= .7)
            {
                grades = "C";
            }
            else if (percent >= .6)
            {
                grades = "D";
            }
            if (percent >= .0)
            {
                grades = "F";
            }
            else
            {
                 Console.WriteLine("WRONG!!");
            }
        }               
        grades = Console.ReadLine();

        //Outputs
        Console.WriteLine(firstName); 
        Console.WriteLine(lastName);
        Console.WriteLine(points);
        Console.WriteLine(percent.ToString("P"));
        Console.WriteLine("so they made an" + grades);

        //Closing statement
        Console.WriteLine("Goodbye");
        Environment.Exit(1);
    }
}

}