How to put two formulas in one cell

Hi,

I want to do something like a bank extract, so if I put a value on the debit (D) cell, I want the cell from the balance (B) after de debit to be E (cell from balance before movement) minus D. But if I put a value on the Credit (C) cell, I want B, to be E+ C.

So basically i want a - and + formula, depending where I put the value

Thanks