Analytics Software

I would like to see the difference of a customer's spending in the current Vs. previous period. So each row contains the data of one customer. And my first field (column A) lists out the customer's spending in the current period and second field (column B) for the previous period. I tried to calculate the difference in my third field (column C).

However, since some of the cells in column A are "null", the respective cells in column C were also shown as "null".
I tried converting column A's null cells into 0 using ifnull. But the respective cells in column C still cannot do the subtraction.

Please help! Thanks in advance.