a simple excel, I have 4 columns in my excel

A is for checking
B is the price or product
C is profit
D is cost

what i want to do is simple, I set formular in C & D, once a user
input either C or D, the another cell will show the different. i.e.

if C is input, D=B-C
if D is input, C=B-D

then A is checking purpose, if either C or D is inputted, it will
shown "Y"

but if I use IF(D0, B-D, "") it will introduce cyclic reference.

any solution for this?
Thanks.