Diophantine Equation Proof: Show that if n=ab-a-b, then there are no nonnegative solutions of ax + by = n

Diophantine Equation Proof: Show that if n=ab-a-b, then there are no
nonnegative solutions of ax + by = n

Let a and b be relatively prime positive integers and let n be a positive
integer. A solution (x, y) of the linear diophantine equation ax + by = n
is nonnegative when both x and y are non-negative. Show that if n=ab-a-b,
then there are no nonnegative solutions of ax + by = n.
Not sure where to begin for this proof question. Anyone got any ideas?