Shared variables in Main report and its sub-reports?

Hi Experts

I wantto know how to use a shared variable in a main report and its subreports. I have create a shared variable in main report coding is given below.

whileprintingrecords;

shared numberVar inv_amt;

if doc_amt> 100000 then

inv_amt := inv_amt + doc_amt;

inv_amt

I placed this formula in detail section it works properly. now i want to show the last total value in sub report also How I can do this?

Regards

Gorge