Can you use If Then logic in a calculated field?

I have table of data that is downloaded from an external source. The amount
field is all positive numbers. The adjacent field indicates if the amount
represents a debit or credit value.

I would like to set up a calculated field that to sum the amount field but I
need to have the debit and credits sum to a net number and not a straight sum.

Any thoughts on this? I'm trying to avoid manipulating the entire table to
change the sign on the credit values.

thanks