Cumulative value Based in other Measure

Hi!

I trying to calculate a Average Headcount (basically is the sum of Monthly HeadCount / number of months). The Monthly HeadCount is the measure that calculates a total value for the month.

for example:

                                      SET   OUT  NOV  DEZ    JAN
Monthly HeadCount    400   430   450    445    480
Average Headcount     400   415   427    431    441

• October is (430 + 400)/2.
• For November, is (450+430+400)/3.

does anyone have any idea how to calculate this measure?

Thanks!