change formula from addition to subtraction visa/versa

I want to be able to change from add or subtract by means of a formula or cel.

so if D1 changes to a + or a - so should the equation in C1 add or subtract.

A1 = 1
B1 = 0
C1 = A1 + B1 (i want this to change if D1 changes)

D1=IF(A1<=B1,"-",(IF(A1>B1,"+")))