ms access training - If statements

I am trying to set up a statement that does the following:
Column 1 - has data
Column 2 - may or may not have data
Column 3 - is to contain the IFf statement which will check columns 1&2 and if 2 is blank, insert the contents of 1. If not, insert the contents of column 2.

Currently I can get it to insert the contents of either one or 2 at a time but not pick from both.