Question about using a SUM component inside an IF cell

Hey.
I’ve been working on a spreadsheet for calculating salary. The idea I had was to use an IF function to work out overtime. I’ve supplied the function below.

=IF(L6>160;“SUM(L6-160)”;IF(L6=160;“Full time”;“Part time”))

L6 Here is the cell where the hours for each month are added up. If they add up to over 160 hours for one month I want the excess hours to be separated to have a different rate applied to them, by taking the full hours minus 160, or print out “Full time” or “Part Time” as appropriate.

Currently it puts out the statements, but i says “L6-160” instead of what that number should be.
I’m looking for some input as to how I might fix this. I hope my explanation makes sense.

LibreOffice 4.2.8.2

Ubuntu Gnome 14.04

If you post the same question in more than one forums, please cross-link the posts.
See http://en.libreofficeforum.org/node/15525#comment-50076

Change to =IF(L6>160;L6-160;IF(L6=160;“Full time”;“Part time”))
When you enter something in quotation marks, it’s considered as text and therefore the formula isn’t executed.