SUM time and disply it

Question

hi

i have mysql table (arret) , i want to sum (duree) it's time column (HH:MM:SS)

i tried this 

string query = "SELECT  SEC_TO_TIME( SUM( TIME_TO_SEC( `duree` ) ) )   FROM arret ;";
            cmd = new MySql.Data.MySqlClient.MySqlCommand(query, conn);

            try
            {
                conn.Open();

                int  s = cmd.ExecuteNonQuery();
                label1.Text = s.ToString();


            }

            catch (Exception ex)
            {
                MessageBox.Show(this, ex.Message);

            }
            conn.Close();

 it's giving me (-1)

my table contains 00:09:00

00:12:32