Conditional formatting with formula

Going mad trying to format a spreadsheet that details availability of invigilators, where each row is a different time slot.
I have a column of how many people I need and a column of how many people are available. I want to highlight the cells where I have LESS people available then I need. I know I can easily do that as a highlight cell rule, but that only works for one cell at a time... Is there a way I can apply this rule to the whole column?
Thanks in advance - it may be a super obvious fix but I'm not tech savvy enough for this, despite what they say about the younger generations... :)

There are no formulas or anything in it, not really a useful sample. Ignore the colour coding. It's Columns C and D.

Thanks, knew it'd be a simple solution, thought I'd already tried that!!