how to subtract 2 fields in my form

From: Dan (anonymous_at_discussions.microsoft.com)
Date: 09/01/04 

Date: Wed, 1 Sep 2004 07:00:36 -0700

i have a form with 2 fields one called est value with an
amount say of $100,000 and another field called priority
liens with an amount of say $10,000 i enter both these
amounts in separate fields and then i want to tab to the
next field and have it subtract these two and put the
amount in this new field automatically is there a way
to do this?