If condition in excel

[sharmaap]

I need to get If condition formula such that, if user has entered data in any 2 rows say a1 and b1, and if user try to enter data in c1 or d1, the user should get error.

User is only allowed to enter data in any 2 rows from a1,b1,c1 and d1. it can be b1 and c1, it can be a1 and d1, if user tries to enter data in third row, he should get error.

 

[p45cal]

Try entering data into the green shaded area of the attached.