subtracting max value from min value

Hi Experts,

I am just learning Crystal reports 2013 and having problems with a formula to subtract Minimum value from Maximum value at the report footer:

WhilePrintingRecords;

{CDBPoint.Name}= "FLOW_TOT_ABBY"

Maximum ({CDPPoint.Name}) - Minimum (CDPPoint.Name})

I get the Maximum and Minimum values at the report footer using running total fields RTotal0 is Maximum while RTotal1 is Minimum.