Adding a total to a NAV report

Hi,

I am currently editing a Packing List report in the NAV development environment (with report builder). The last thing I need to do is to add several totals to the report (for example total weight).
The weight of each line in the packing list is calculated by Quantity*(Weight of 1 item). I cannot manage to calculate the total weight, since the weight of each line is not stored anywhere (so something as easy as SUM(weight) does not work).
Is there maybe some way to keep track of a running total with a few lines of code, which I print at the bottom of the report in the end?

Thanks in advance!