Pivot table and running total
 

I have a table with Columns: Region, Client, Date, and quantity.
Using a pivot table I want to see per Region the quantity each Clients
bought or sold.
The problem I have is I can't figure out how to view a running total per
region.
You can view at this URL "http://members.freemail.ch/paulg/pivottable.gif"
a picture of how I would want to see it (in yellow) and the table it comes
from.
Actual data has at least 20 different regions and 60 or so clients.
Any help greatly appreciated.
Thanks