Gridview : how to add row for sum total after some rows

Hello 

I want to add after some rows (not fixed) sum of all the rows in the gridview 

e.g. 

Gridview

DEPT EMP Sal

100   1     1000

100    2     1000

100    5      200

Total  - --     2200

200    6   1200

200    3    2300

Total   ---   3500

300   8    1000

300  4     1020

300   9     2000

300   10    1000

Total ----     5020

This way i want result 

Can u help me ?