sum of listgrid

I'm a beginner in smartclient. I'm developing a window on Openbravo and I want to display the sum of totals in few seconds. My sum displays just if I go in the end of page. I sent below the file and a screenshot. Can you help me please?
Thank you in advance!

Attached Images

screenshoot.jpg (82.2 KB)

Attached Files

main.js (6.2 KB)