Count previous months

If i have  a  pivot or table  with Year / Month  ; 
I want to count the previous number of months  present so not just the month number  
as some do not have any data so are not present 

so if i had 
Jan sales 1

March sales 2 
May sales 3    

so the best i have so far is ; 

Count Previous Months:=VAR Myear =
    IF( HASONEVALUE( 'Calendar'[Year]),VALUES( 'Calendar'[Year]))
VAR Mmonth = 
    IF (HASONEVALUE( 'Calendar'[Month Number]), VALUES('Calendar'[Month Number]))
RETURN
CALCULATE (
    DISTINCTCOUNT ( 'Calendar'[Month Number]),   
      FILTER(ALL('Calendar'), 'Calendar'[Month Number]<= Mmonth && 'Calendar'[Year] = Myear))

so this counts the months regardless of whether there are 'sales' present 

Richard