How to write this calculated field?

Hi,

I created an Actual/Fcst measure under Actual table and am trying to write an calculated field like this:

For 2017 Actual

If Actual Volume is not available (e.g. 4/6/2017....12/31/2017, use 2017 FCST.

If Actual Volume is available, use 2017 Actual.

Could anyone teach me how to write this calculated field? I am currently using tableau v9.2

Two datasets:

1. Actual Table: daily volume starting from 01/01/2015-04/04/2017(till now).

2. Forecast Table: daily forecast starting from 01/01/2017-12/31/2017.

Thanks!