Using Multiple If statements in a cell

[Guygettinby]

Help!!!!!!
Please could someone give me a pointer.
I want a cell to take away a set amount if a different cell from a range has the letter "S" in it. If more than one cell has "S" in it I want it to take away a further amount. I can get it to work with just one "S" in any of the boxes from the range but it wont work with more than one. The formula I have at the minute is:
=SUM(Sheet1!B5:F5,IF(Sheet1!B6="S",-6,IF(Sheet1!C6="S",-7.5,IF(Sheet1!D6="S",-7.5,IF(Sheet1!E6="S",-7.5,IF(Sheet1!F6="S",-7.5,IF(Sheet1!B6="UH",-6)))))))