SSRS column running value plus opening value

Hi All,

Im trying to do report that calculate the running value across columns.  Running value itself works fine. But I want to add another field from along with the running value. but the formula doesn't work. example below

Data

                                                                purchases

                opening stock               Feb           March          April

Item A      10                                5                 4               6

Item B      20                                10               5                1

Item C      30                                20               5               10

Report should look like this

                                                               Net stock by month

                opening stock               Feb           March          April

Item A      10                                15              19             25

Item B      20                                30               35            36

Item C      30                                50               55             65

groups

1. row = by item

2. column = by month

I applied formula like = runningvalue(sum(purchases),sum,"group_item")+opening stock

any ideas to get the report I want?

thanks