62097 Auto Logout prompt

Hi expert programmers, I would need a simple script which can pop up a javascript box saying "DO YOU WANT TO CONTINUE USING THIS SESSION 'yes','no'" after the page HAS BEEN IDLE FOR 15 minutes. If the user click 'no', this script will kill of the session of the user..(therefore logging out). This page will be done is ASP, and preferrably the script i'm looking for should be in Javascript. Guaranteed payment when the work is working under my own codings. Thanks for reading.