How to display the product of two rows of datagridview in another row. Absolutely like the MS excel .

I'm trying to find a solution to a problem. I have added two columns of datagridview to a project of VB.net. I want - when I give a number in the second column, the product of the number in my first column with this number will be displayed in the third column(
Multiplication). Like working in MS Excel.
Will it be possible? Although the correct result is being added to my database but the data is not being displayed in Gridview. I want to see the product of two columns in the datagridview.
I would have benefited if an experienced person had wasted his precious time and assisted me in this matter. Thanks

 Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
  Try
  For Each row As DataGridViewRow In DataGridView1.Rows
   Dim select1 As Boolean = Convert.ToBoolean(row.Cells("checkboxcolomn").Value)
   If select1 Then
  Dim cmd As New SqlCommand("insert into tbl_Purchase (Product_Name,PurChase_Rate,Qty,Value) values(@d2,@d3,@d4,@d5)", con)
 cmd.Parameters.AddWithValue("@d2", row.Cells("Product_Name").Value)
 cmd.Parameters.AddWithValue("@d3", row.Cells("Purchase_Rate").Value)
 cmd.Parameters.AddWithValue("@d3", row.Cells("Qty").Value)
cmd.Parameters.AddWithValue("@d5", row.Cells("Qty").Value * row.Cells("Purchase_Rate").Value)
                   
con.Open()
cmd.ExecuteNonQuery()
con.Close()
 End If
 Next
 MsgBox("Data added successfully")
Catch ex As Exception
MsgBox(ex.Message)
Finally
If (con.State = ConnectionState.Open) Then
con.Close()
End If
End Try
End Sub

Private Sub DataGridView1_CellValueChanged(sender As Object, e As DataGridViewCellEventArgs) Handles DataGridView1.CellValueChanged
Dim cell1 As DataGridViewCheckBoxCell = DataGridView1.Rows(e.RowIndex).Cells(0)
cell1.Value = True


 Private Sub frm_purchase_Load(sender As Object, e As EventArgs) Handles MyBase.Load
 Dim checkboxcolumn As New DataGridViewCheckBoxColumn()
 checkboxcolumn.HeaderText = "*"
 checkboxcolumn.Name = "checkboxcolomn"
 DataGridView1.Columns.Insert(0, checkboxcolumn)

 Dim cmd As New SqlCommand("select a.Product_Name,a.Purchase_Rate from tbl_Pro a", con)
 Dim da As New SqlDataAdapter(cmd)
Dim dt As New DataTable
dt.Clear()
da.Fill(dt)
DataGridView1.DataSource = dt

            dt.Columns.Add("Qty")
            dt.Columns.Add("value")
 End Sub

Like 
Product_Name    Purchase_Rate    Qty     Value
    p1		          5		      2	      10