Excel - is there a BUT function?

I am currently trying to figure out a formula; I have the following: =IF(G18>109,"A*",IF(G18>103,"A",IF(G18>85,"B",IF(G18>67,"C","F"))))

This allows me to work out grades for my students in my French class but I want to add another part to the formula... I want it so that if C18:F18 says "INC", then the cell this formula is in says "inc" for an incomplete course.

Is that possible?