Count the people, wait for the "shift" and count them again.
http://members.tccoa.com/lastmrk/Images/countpeople.gif
http://members.tccoa.com/lastmrk/Images/lastmrk2.jpg
http://members.tccoa.com/lastmrk/Images/countpeople.gif
http://members.tccoa.com/lastmrk/Images/lastmrk2.jpg