how to find sum of a group?

Hi all,
I'm currently doing a project regarding crystal report.
I want to find the sum of SubTotal that displayed in each group.
For example,
I have 2 groups named Normal and Best Buy.
In each group,they have their own group that named 0% and 10%.
so now I have to sum the Quantity of 0% and 10% in SubTotal.
After that,I need to sum up the SubTotal of 0% and 10% as GrandTotal.
 
In my report,I using Insert->Summary to get the SubTotal of 0% and 10%.
The problem now is how can I sum up the SubTotal?
Because that is not a variable in the report.