C# winform sum datagrid columns

<pre>private void LoadDataGridView(DataTable data)
        {
            // Data grid view column setting            
            dataGridViewMembers.DataSource = data;
            dataGridViewMembers.DataMember = data.TableName;
            dataGridViewMembers.AutoResizeColumns(DataGridViewAutoSizeColumnsMode.AllCells);

            // Sum Rich Text Box
            //
            //
            double sum = 0;
            for (int i = 0; i < dataGridViewMembers.Rows.Count; ++i)
            {
                sum += Convert.ToDouble(dataGridViewMembers.Rows[i].Cells[6].Value);
            }
            rtVacSum.Text = sum.ToString("N2");

            //// double sum = 0;
            for (int i = 0; i < dataGridViewMembers.Rows.Count; ++i)
            {
                sum += Convert.ToDouble(dataGridViewMembers.Rows[i].Cells[7].Value);
            }
            rtCompTime.Text = sum.ToString("N2");
        }