hi,

Joseph, you modified program upto:
b = (z - !z)*(a - b);

Now do some more steps forward.
Because variable zmay be only 0 or 1 (remember z = a>b), so (z-!z) may produce only 1 or -1.
As result, now we have two ways:
if a>b then b = a-b;
if a<=b then b = b-a;

Regards,
Oleg