SQL counting the values from two columns and showing the result in other column at the same table

SQL counting the values from two columns and showing the result in other
column at the same table

I have two columns in one table, lookin' like this:
CREATE TABLE Invoices
(
ID_Invoice...
....
UnitPrice decimal (15,2) NOT NULL,
Discount (15,2) NOT NULL,
)
So I wanna add a new column in this table which count the final price -
UnitPrice-Discount=TotalCost? Please help :(