Hi Rajesh,
You need to use SUM function under category Statistics. Find the below document where you can find screenshots with examples:
http://wiki.sdn.sap.com/wiki/display/XI/Standard+Functions+in+PI+7.0
SUM:
R = Sum of values X1 to Xi of a context.
For example I want to take the sum of payment:
Regards,
Nabendu.