About subtotal of report

[Derek]

Hi all,

I want to create a report group by months and with a
running sum. for example, the data will be grouped by Jan,
Feb, Mar, and in each group, there's a subtotal to
calculate the YTD sum. In Jan, the YTD sum will be Jan, in
Feb, the YTD sum will be Jan+Feb, and in Mar, the YTD sum
will be Jan+Feb+Mar. Can anyone please tell me how to do
that? Thanks a lot!