In C# How to read data from table and after some calculation save those data in another table?

I an new in C#.I am trying to read data from a table named 'tblAddResult'. In this table I have some columns and i want to make average of 3 columns of tblAddResult and save it to a new table named tblResult. SOme other calculations also wanna do there like Taking 50% of a data of a column of tblAddResult.I did this in this way(code placed below) but i am getting an error "invalid attempt to call read when reader is closed". I dont even know is this the right way to do it if not can anyone help me with the write way or any suggestions regarding this way. I am really helpless for this problem.

     private void button1_Click(object sender, EventArgs e)
       {
        con.Open();
        cmd = new SqlCommand("SELECT   tblAddResult.* FROM  tblAddResult", con);
        SqlDataReader sdr = cmd.ExecuteReader();

        while (sdr.Read())
        {

            String subject = sdr.GetString(2);
            int january = sdr.GetInt32(3);
            int february = sdr.GetInt32(4);
            int march = sdr.GetInt32(5);
            int average = (january + february + march) / 3;
            int average40= average*40/(100);
            int marks = sdr.GetInt32(6);
            int marks50 = marks * 50 / 100;
            int WorkingDay = sdr.GetInt32(7);
            int Attandence = sdr.GetInt32(8);
            int Attendence10 = Attandence * 10 / 100;
            int totalMarks = average40 + marks50 + Attendence10;
            string grade = "" ;
            if (totalMarks < 51) { grade = "C"; }
            else if (totalMarks < 61) { grade = "B"; }
            else if (totalMarks < 71) { grade = "A-"; }
            else if (totalMarks < 81) { grade = "A"; }
            else if (totalMarks < 91) { grade = "A+"; }
            else if (totalMarks <= 100) { grade = "A++"; }
            con.Close();
            con.Open();
            SqlCommand comnd = new SqlCommand("INSERT INTO tblResult (Subject, [Full Marks], January, February, March, [Average Class Perfomance], [Earned Marks], [Working Day], Attendence, [Cls Attendence(40%)], [Exam Perfomance(50%)], [Attendence(10%)], [Marks(%)], Grade)VALUES  ('"+subject+"','"+"100"+"','"+january+"','"+february+"','"+march+"','"+average+"','"+marks+"','"+WorkingDay+"','"+Attandence+"','"+average40+"','"+marks50+"','"+Attendence10+"','"+totalMarks+"','"+grade+"')",con);
            comnd.ExecuteNonQuery();
            MessageBox.Show("Successfull!");

        }