How to sum a field in the data table

Question

 SqlConnection cn = new SqlConnection("Data Source=.;Initial Catalog=Demo_eLogsVA4;Integrated Security=True");

        //Write a select Query

        cn.Open();

        string q = "Select Invoice_Code,Amount,Tax,Net Where Invoice_Code='INV991'";

        DataSet dset = new DataSet();

        SqlDataAdapter da = new SqlDataAdapter(q, cn);        

        DataTable dtable = new DataTable();

        da.Fill(dtable);

        //Bind grid.

        GridView1.DataSource = dtable;

        DataRow[] dr = dtable.Select("Sum(Net)");

        GridView1.FooterRow.Cells[0].Text = Convert.ToString(dr[0]);

        GridView1.DataBind();

        cn.Close();

the above is my gridbind method. My table name is dtable, from that dtable i want to sum the amount and net field in the datatable(dtable) and where i want to include the sum code ,Help please