Formula Field does not work with basic operator

Formula Field does not work with basic operator

I can't seem to find anything relating to this in the documentation or in the forum for what I thought would be a pretty basic function...  Even an example in the documentation would have helped....

I have 2 x Currency Fields.

I am trying to create a Formula Field which is the result of adding the two fields together.

1. How could I possibly get something so basic wrong?
2. How could something so basic have no documentation attached if the "logical way" of doing it does not work???

Formula Return Type: Currency
Decimal Places: 2

Select field: Airfares & Click "Insert"
Select operator: + Add & Click "Insert"
Select field: Accommodation & Click "Insert"

Click "Check Syntax" and I get "Syntax Error for Given Formula"

How could something so basic have any more requirements that 2 x fields and 1 x operator???  What could I possible be doing wrong???