SSRS Sum group by

Hi,

I have a report like below:

Cat | Sub Cat | Count
 A     
        B         50
        C         40
        D         30
        B         60

Is it possible to sum up the sub categories?

Cat | Sub Cat | Count
 A     
        B         110
        C         40
        D         30

1. tried giving the expression for count as =Sum(Fields!Count.Value,"Category") but this gives me the total count of (50+40+30+60)
2. by giving  =Sum(Fields!Count.Value,"Sub Category") gives me "the value expression for the textrun contains an aggregate function..."

Please help.

Thanks