getting Exception as converting data type nvarchar to numeric

public partial class WebForm9 : System.Web.UI.Page
    {
       
        SqlCommand cmd = new SqlCommand();
        SqlCommand cmd1 = new SqlCommand();
       DataTable dt = new DataTable();
        DataRow drr;
        public string ans_tea;
        public string ans_stu;
        SqlConnection con = new SqlConnection(@"Data Source=.\sqlexpress;Initial Catalog=swatidb;Integrated Security=True;Pooling=False");

        static int i = 0;
        
       
       
           public void Page_Load(object sender, EventArgs e)
        {
       
            con.Open();
            string s1 = "select student_id from Student where user_name='" + Session["sid"] + "'";
            SqlCommand cmd = new SqlCommand(s1, con);

            SqlDataReader dr;
            dr = cmd.ExecuteReader();
            if (dr.Read())
            {
                Label4.Text = dr[0].ToString();
                dr.Close();
            }
            String tea_id = Convert.ToString(TextBox2.Text);
            string commandText = "SELECT * from Question WHERE Teacher_id=@tea_id";
            using (SqlConnection connection = new SqlConnection(@"Data Source=.\sqlexpress;Initial Catalog=swatidb;Integrated Security=True;Pooling=False"))
            {
                using (SqlCommand command = new SqlCommand(commandText, con))
                {
                    command.Parameters.AddWithValue("@tea_id", tea_id);
                    SqlDataAdapter da = new SqlDataAdapter(command);
                    DataTable dt = new DataTable();
                    da.Fill(dt);
                    drr = dt.Rows[i];
                    
                }
            }

             public void Button2_Click(object sender, System.EventArgs e)
        {
            
          //  Label2.Text = Convert.ToString(dr[2]);
           
            //Label3.Text = Convert.ToString(dr[3]);
            //TextBox3.Text = Convert.ToString(dr[2]);
            //TextBox4.Text = Convert.ToString(dr[3]);
                                 if (i == (dt.Rows.Count - 1))
            {
                Response.Write("Last record !");
            }
            else
            {
                //i = 0;
               i++;
             
            }
                                // drr = dt.Rows[i];
                                 Label3.Text = Convert.ToString(drr[0]);
                                 Label2.Text = Convert.ToString(drr[2]);
 
                      //Label2.Text = Convert.ToString(dr[2]);
                                // i = 0;
                                 con.Close();
       }

       }