Timesheet - overtime as a negative and positive

please see attached timesheet.in P12 it shows flexi.Staff work 37 hours per week and occasionally worked overtime/flexi.in P12 it works out if more than 37 hours are worked per week, however sometimes staff work less tahn 37 hours per week and owe the company time back.how would i be able to show in column P12 if they owe time back as in worked 35 hours so that week that owed 2 hours that needed working back ?