C# Creating a Grading Program

So I am trying to create a gradeing program where the user inputs a number between 1-100 on subjects; math, gym and classroom and the average number comes up that gives the student a grade between A-F.

I am stuck at looping, i dont know how to code so that if I type in an average of ex 80 points the grade B comes up. It comes up unreachble code detected.

  static string GradeMe(int average)
 {
        const int grades1 = 90;
        const int grades2 = 75;

       if (grades1 >= 90 && grades1 <= 100) 
           { 
              return "Grade A"; 
           } 

     (???->) else if (grades2 >= 75 && grades32 <= 89) {return "Grade B";}
    }

static int CalculateAverageGrades(int math, int gym, int classroom)
  {
        int sum = math + gym + classroom;
        return sum / 3;
    }
      static void Main(string[] args)
    {
        Console.WriteLine("math: ");
        int tenta = Convert.ToInt32(Console.ReadLine());
        Console.WriteLine("gym: ");
        int dugga = Convert.ToInt32(Console.ReadLine());
        Console.WriteLine("classroom: ");
        int projekt = Convert.ToInt32(Console.ReadLine());

        int medeltal = CalculateAverageGrades(math, gym, classroom); //hämtar calculate metoden

        Console.WriteLine("medel: " + medeltal);

        string grade = GradeMe(medeltal);
        Console.WriteLine(grade);