need help in correcting the formula

[Guest]

C2=IF(A2=0,C1+B2,IF(B2=0,C1-A2,IF(A2&B2=,"0","ERROR")

what im trying ti do is to make the formula to show an error message when
(A2 and B2) both cells have been entered with data at the same time. that is
to say i want the results to be generated from either A2 or B2, not both.
when both cells (A2 and B2) are entered with data it should show "ERROR".
this is the best way i could explain. please help.

 

[JE McGimpsey]

If the cells will contain at least one user entry:

=IF(COUNT(A2:B2)=2,"Error",SUM(C1,B2,-A2))

If you want to generate the error message if neither A2 or B2 are filled
in, or both are filled in:

=IF(MOD(COUNT(A2:B2),2)=0,"Error",SUM(C1,B2,-A2))