in the Field Explorer fine 'Running Total Fields'
Right click on it and select NEW
a window opens up
RT name =FIrst value (or whateer you want)
field to summarize = select the column A field for this
Type of SUmmary = select SUM
Evaluate section
select On change Of Group toggle option and then in the pick list sleect the group name here
Reset section
select On change Of Group toggle option and then in the pick list sleect the group name here
Save it.
Drag it and drop it on the group footer