Data Validation STOP Error Function

Hi all,

Once again I am here seeking your help. I tried google & forum search but couldn't find much help. Here is my question. Value of A1 & B1 is entered by user & C1 is (B1-A1). But B1 cannot be greater than A1, for that I used Data Validation STOP Error Function, it worked really fine but it has a loop hole. Eg. : If user enters 2 in A1 & 3 in B1 it will show error (this works fine). But when user deletes 2 from A1 & enters 3 in B1 & then enters 2 in A1 this does not throws any error ideally it should. Thanks in advance.