How to format a formula field type to add 2 fields with numerical values?

[Bill Mills]

I have 2 fields, field1 and field2, that are formula field types, and have numerical values.

I have a 3rd field where I'm trying to add field1 and field2 together to give me a single number.

E.G.:

- the value of field1 is 4 and the value of field2 is 1 (field2 may also be empty). I'd like the 3rd field to add 4+1 (or 4+0 if field 2 is empty), for a result of either 5 or 4. 

I've tried many different expressions without luck.  Please help!

Thanks