Creating a measure for Delta

I wrote this code to calculate the delta ( Diffrance )between the current year for the current month from the previos year for the same month BUT unfortunately it gives me sysntax error "," Can you please help to figure out where is the problem and how to solve it 

Delta =
VAR CurrentMonth = MONTH(TODAY())
VAR CurrentYear = YEAR(TODAY())
VAR PreviousYear = CurrentYear - 1
RETURN
CALCULATE(
SUM(Sales[Amount]),
SAMEPERIODLASTYEAR('Date'[Date]),
YEAR('Date'[Date]) = PreviousYear,
MONTH('Date'[Date]) = CurrentMonth
) - CALCULATE(
SUM(Sales[Amount]),
YEAR('Date'[Date]) = CurrentYear,
MONTH('Date'[Date]) = CurrentMonth
)

Thanks.