how to get difference of 2 group header

Hi

I am developing report in crystal report

in that, i have grouped with titlename

titlename1 1000

-----------------------------------------

800

200

titlename2 700

--------------------------------------------

500

200

titlename3 300

---------------------------------------------------

titlename4 400

200

200

I want to get in titlename3(1000-700)=300 from (titilename1-titlename2)

group total 700,300 I got from summary field.Please help me.

Thanks and Regards

Rama