formula

Is it possible to create a formula that shows a 0 when the total is a minus after a calculation ?
Let me explain .
Cells a1 to a12 is Jan to Dec
Cells c1 to c12 is monthly use
Formula in say cell c2 is ( +a2-a1) , cell c3 is ( +a3-a2 )and so on across to c12 which will then give a negative value in the following c cell ( cell c3 ( +a3-a2 ) ) then shows a negative value in cell c4 as the months go by . so the formula that I am after is say where March figure ( a3 ) is inserted giving a monthly use in c3 , and there is no entry in April ( a4 ) the negative figure is shown in cell c4 . I would like this c4 cell to show 0 .
May I thank you in anticipation of your assistance

Franky