here is a post to answer your question. Please read it. If you have questions leave a comment below.
The exception in your case is to use a minus (-) in place of the addition (+) in the advice at the link. All else applies the same principles.
The answer will be the difference.
http://web2.0calc.com/questions/viewtopic.php?f=2&t=8648&p=18361&hilit=lowest+common#p18361 <--highlight and right click, select goto.
+1313 
