Need help with a nested IF with an ISNA (I think)

Hi all, I've attached a sample workbook. I am trying to create one main "master" column of numbers from 7 columns with data in it. Essentially, column 1 has priorty, so if there is something in there, I want to use that. But if there is an #N/A in there, I want to use what is in column 2. If there are #N/A's in columns 1 and 2, then I want to use what is in column 3....and so on.

I was thinking a combination of IF and ISNA would work, and I can get it to work for 2 columns, but I'm having trouble getting it to work for all 6.

Sample file attached. Thanks a lot for your help.