I want to make a cell only validate an entry if another cell equals a value.
I'm using DATAVALIDATION and selecting CUSTOM. This is my formula:

=IF(NOT(C5="R"),"X","")

I want this cell to recognize if Cell C5 contains ONLY the letter "R" it is
to accept ONLY the letter "X" as an entry. If cell C5 doesn't contain ONLY
the letter "R" it is to accept ANY entry.