Return a Fail, Pass, Merit or Distinction

Hi
I have a collation spreadsheet that collects grade from other sheets. I am trying to work out the grade for a specific unit of work to give me either "F", "P", "M" or "D" for fail, pass, merit and distinction respectively.
I've assign a number to each grade level:
"F"=0
"P"=1
"M"=2
"D"=3

Column S adds the assigned numbers to give me a total. I than used an IF statement to work out the grades eg.
=IF(S4<6,"F",IF(S4=6,"P",IF(S4<12,"P",IF(S4=12,"M",IF(S4<18,"M",IF(S4=18,"D","F"))))))

This works fine if the students have a "P","M" or "D" grade in each of the previous columns. However, in row 22 it does not work because enough points have been scored for the formula to return "P". This is an incorrect grade because not all the work has been completed. I want it to return "F" in column T if any of the columns have an "F" in it.

How can I make this work please?