Running Numbers

 Hi Every one,

I have created a combined analysis report to show Opportunity having activity and account. I have used Opportunity and activity subject area. The report is working fine but currently client requires one more Column with name "Row Number", which displays running numbers(like 1,2,3,4 etc.) . I am using RSUM(1) function in the Row number column and when i include it in Pivot Table report displays opportunity records multiple times . Like number of opportunity records with account seperately and opportunity activty record seperately.Without Row number column it works fine. So is there any other way or formula  can be used