how to add new row sum of column in excelsheet

Question

I am working with excelsheet

I am trying to add sum of column in new row but not able to add sum of column

 

Current Output: 

 

 

 

Expected Output(I want to below output):

 

 

 

see code: 

private void AddExcelSheet(DataTable sDT, ExcelWorksheet excelWorksheet)
{

    DataTable dtCustomDetail = new DataTable();
    dtCustomDetail.Columns.Add("Id", typeof(string));
    dtCustomDetail.Columns.Add("Name", typeof(string));
    dtCustomDetail.Columns.Add("Cust Name", typeof(string));
    dtCustomDetail.Columns.Add("Cust Qty", typeof(int));

    if (sDT != null && sDT.Rows.Count > 0)
    {
        foreach (DataRow Dr in sDT.Rows)
        {
            DataRow DRowDetail = dtCustomDetail.NewRow();

            DRowDetailCO["Id"] = Dr["Cust_Id"].ToString();
            DRowDetailCO["Name"] = Dr["Name"].ToString();
            DRowDetailCO["Cust Name"] = Dr["CUST_NAME"].ToString();
            DRowDetailCO["Cust Qty"] = Dr["Cust_QTY"].ToString();

            dtCustomDetail.Rows.Add(DRowDetails);

        }
    }

    excelWorksheet.Cells[2, 1].LoadFromDataTable(dtCustomDetail, true); //here load my table

    excelWorksheet.Cells[].Formula = "=SUM(D3:D" + (6) + ")";   //here I am trying to add some of column in new line but I am stuck

why I am using foreach loop because I have not idea how many data in table so I use foreach loop 

 

now I want to add a new column for some but I have no idea 

 

please help