Shopping Cart, How Do I Subtract The Quantity Purchased From The Stock In Database?

Im making a shopping cart website for a school project in ASP.net with VB. I need help subtracting the quantity purchased (its saved in a session) from the stock number saved in a database.

I know this:UPDATE inventory SET stock = stock - <quantity_purchased> WHERE id = <inventory_id>

But I dont understand how to get the quantity purchased from the session to <quantity_purchased>. I tried putting the name of the session there and I got an error, i tried saving the session into a dim didnt work either.

 

<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>"

SelectCommand="SELECT [stock] FROM [product]" InsertCommand="INSERT INTO [product] ([stock]) VALUES (@stock)" UpdateCommand="UPDATE product SET stock = (stock - @Quantity) WHERE (productID = @productID)">

<InsertParameters>

<asp:Parameter Name="stock" Type="Int16" />

</InsertParameters>

<UpdateParameters>

<asp:SessionParameter Name="Quantity" SessionField="Quantity" Type="Int32" />

<asp:SessionParameter Name="productID" SessionField="productID" Type="Int32" />

</UpdateParameters>

</asp:SqlDataSource>

 and I have than in my VB code on submit : SqlDataSource1.Update()