mask : mask extrat from polygon after rastersize

EVI-1-2000  >- raster
EVI-2-2000
....
EVI-6-2000

 Mask*EVI : value of raster

> sum(as.array( Mask*EVI ),na.rm=T)

[1] 3996633

> sum(mask,na.rm=T)   # number of pixels

[1] 5522011

> sum(as.array(mask*EVI),na.rm=T) / sum(mask,na.rm=T) # mean