помогите

class Student : IComparable
    {
    string lastName, firstName;
    int groupNumber;
    static int[] progress = new int[5];

    public Student() { }

    public static int[] Progress
    {
        get
        {
            return progress;
        }
        set
        {
            progress = value;
        }
    }

    public Student(string lastName, string firstName, int groupNumber, int[] progress)
    {
        this.lastName = lastName;
        this.firstName = firstName;
        this.groupNumber = groupNumber;
    }

    static double MediumGrade(int[] progress)
    {
        double medGrade = 0.0;
        for (int i = 0; i < progress.Length; i++)
        {
            medGrade = medGrade + progress[i];
        }
        return medGrade / progress.Length;
    }

    double medGrade = MediumGrade(Progress);


    public int CompareTo(object obj)
    {
        if (obj == null) return 1;
        Student s1 = obj as Student;
        if (s1 != null)
        {
            return this.medGrade.CompareTo(s1.medGrade);
        }
        else throw new NotImplementedException ();
    }
}


class Program
    {


        static void Main(string[] args)
        {
        Student s = new Student("Иванов", "Иван", 3115, new[] { 5, 4, 5, 5, 3 });
        Student[] st = new Student[3];
        st[0] = s;
        Student n = new Student("Нотова", "Ната", 3111, new[] { 5, 5, 4, 4, 5 });
        Student k = new Student("Котов", "Котя", 324, new[] { 4, 5, 4, 4, 5 });
        st[1] = n;
        st[2] = k;
        Console.WriteLine(n.CompareTo(s));

    }
}