Formula needed to determine higher figure and subtract lower

[tizwas]

Hi.. I'm an excel newbie and so would be grateful for any help with my question;

a b c

100 50
100 150

I want to get an answer for c. I need to start with col a as that is my base fig. Sometimes the figure in b is lower than a and sometimes it's higher so I need a calculation that determines which is the higher figure a or b and then subtracts the lower figure and displays it in c.
I do not want a negative figure. Does this make sense?

Chris.