Making a cell equal either one of two cells

Hi everyone,

I am having trouble with a formula in excel 2010.
I have a cell (F657) that I want to be equal to either cell F340 or F235, depending on which of these two cells is not blank.
Both F340 and F235 are dependent on other cells (F351 or F247) and are blank if those cells are empty.

Can anyone provide me with the right formula to do this?

I hope my explanation is clear enough.

Many thanks in advance!