How to Subtract 1 From a Orginal Count in an ASP.Net GridView

I have a gridview that contains a count (which is Quantity) where i have a button that adds a row under the original row and i need the sub row's count (Quantity) to subtract one from the orginal row Quantity.

EX: Before button click

Orgianl row = 3

After click

Orginal row = 2
Subrow = 1

Code

ASP.Net

// FUNCTION : Adds a new subrow

protected void gvParent_RowCommand(object sender, GridViewCommandEventArgs e)

{

if (e.CommandName.Equals("btn_AddRow", StringComparison.OrdinalIgnoreCase))

{

// Get the row that was clicked (index 0. Meaning that 0 is 1, 1 is 2 and so on)

// Objects can be null, Int32s cannot not.

// Int16 = 2 bytes long (short)

// Int32 = 4 bytes long (int)

// Int64 = 8 bytes long (long)

int i = Convert.ToInt32(e.CommandArgument);

// create a DataTable based off the view state

DataTable dataTable = (DataTable)ViewState["gvParent"];

for (int part = 0; part < dataTable.Rows.Count; part++)

{

int oldQuantity = Convert.ToInt32(dataTable.Rows[i]["Quantity"]);

string partNumber = dataTable.Rows[i]["ProductDescription"].ToString();

string description = dataTable.Rows[i]["Description"].ToString();

string dateOrdered = dataTable.Rows[i]["GTRI_DateSubmittedtoPurchasing"].ToString();

string estShipDate = dataTable.Rows[i]["Gtri_EstShipDate"].ToString();

string actualShipDate = dataTable.Rows[i]["Gtri_ActualShipDate"].ToString();

string trackingNumb = dataTable.Rows[i]["GTRI_TrackingNumbers"].ToString();

string serialNumb = dataTable.Rows[i]["Gtri_SerialNumber"].ToString();

int oldQuantitySubtract = Convert.ToInt32(dataTable.Rows[part]["Quantity"]);

string curentPartNumbers = dataTable.Rows[part]["ProductDescription"].ToString();

string currentDescription = dataTable.Rows[part]["Description"].ToString();

string currentDateOrdered = dataTable.Rows[part]["GTRI_DateSubmittedtoPurchasing"].ToString();

string currentEstShipDate = dataTable.Rows[part]["Gtri_EstShipDate"].ToString();

string currentActualShipDate = dataTable.Rows[part]["Gtri_ActualShipDate"].ToString();

string currentTrackingNumb = dataTable.Rows[part]["GTRI_TrackingNumbers"].ToString();

string currentSerialNumb = dataTable.Rows[part]["Gtri_SerialNumber"].ToString();

if (partNumber.Equals(curentPartNumbers, StringComparison.OrdinalIgnoreCase) && oldQuantitySubtract > 1)

{

dataTable.Rows[part]["Quantity"] = oldQuantitySubtract - 1;

// Instert a new row at a specific index

DataRow dtAdd = dataTable.NewRow();\

for (int k = 0; k < dataTable.Columns.Count; k++)

dtAdd[k] = dataTable.Rows[part][k];

dataTable.Rows.InsertAt(dtAdd, i + 1);

break;

//dataTable.Rows.Add(dtAdd);

}

// Rebind the data

gvParent.DataSource = dataTable;

gvParent.DataBind();

}