How to add sum to a group in a cross tab report

Hi,

I am using ireport 4.0.1.

I've created a cross tab report which shows sum of student receipts on a monthly basis. Initially, I created the report with one row group (student) and added another group for opening balance (group1).

below is the screen shot of the report

I've created a variable to sum opening balance as

the report has created group1/Detail and group1/month_year sections for opening balance group.

I want to drag sum_op_bal in the group1/Detail section but its not allowing me to do so. I have also tried dragging a Field item in this section but that didn't work either.

Please can someone help with it?

thanks