The [pak] object will help. Also I’d recommend using straight math operators if you only need one of them in your equation, for example use [-] rather than [expr $i1-$i2]. Here’s a patch:

-- Pasted Max  Patch, click to expand. -- 
Copy all of the following text. Then, in Max, select New From Clipboard.
----------begin_max5_patcher----------
345.3ocyTF0SCBCDG+4xmhl9Lag1AiMeyOGlESApaUgVBThnK66tzBU2Lts5
Pm9Ra5cG28+9Utt0CfRjsrZD7F3cP.XqG.XLoM.FNCPEz1zbZsILjnoHgUg7
6c8fTnDzBlw0sUbZt0SIUktgKVeeEKU0W.RHYZfODubldKJPuR5VgqF9FdlI
OxjGmrvlmt5wE4Lko538JaM+USYw5j9drxFkMXxf0dSpWJY8x.wEJjODkPEq
QvU5f144oW7ckArm6znUgJVqoAQSFEUhw5sYjoQGmJy+RpP9NTAeJp7SRiR5
Sv.XvnXR3R8VL4T+oD8KxjKDHiXDYYjowIwmeDI7e8Hx3elvEFP9KXfQHnbt
3yubZZFs8CASsroJ0l0gwX3GsSFqVwETEWJ1KlEGDyFdVFy311DE7rRYmDGj
vQtjbUQQNnn4WUEE5fhzpFe0TDwQEcoLp6vNu2fp.wKi
-----------end_max5_patcher-----------



			

You should read this thread (it’s a bit hidden):
http://cycling74.com/forums/topic.php?id=26095

 
5       7
|        |
|     [t b i]
|  /          |
[expr $i1-$i2]
 |