roll up issue

I have 3 columns, A, B, C. A and B are selections, C is a formula. B has a value of 0 most of the time. When ever, B has a value other than 0, C takes the value of B. If not, C takes the value of A.

Basically, formula for C is

C = (B<>0)B + (B==0)A

The roll up gives me a problem. I have the calculate result as "summation" option checked. It doesn't sum the values in column C. It applies the same formula at the region/district level and gives an incorrect value. Since B has a value at the roll up level it gives the value of B as the result which is incorrect.

How can I make it sum all the values of C?