How to calculate the moving average for binary files?

I have 12 binary (raster) files   
https://echange-fichiers.inra.fr/get?k=k3M2jatJyHy65Cs99G4 .
I would like to calculate the moving average for the 12 values for each
pixel in the 12 files.

For a simple vector we can get a moving average by using this :

            x <- c(1,2,3,NA,NA,4,6,5,6,4,2,5)
           movingmean <- rollapply(x, 3, FUN = mean, na.rm = T,fill=NA)
now I want to do the same but with binary files and I tried:

   files   <- list.files("C:final-2010", "*.envi", full.names = TRUE)
   results <- list()
   for (.files in files) {
       # read in the 12 files as a vector of numbers 
       # that we take the average of
       x <- do.call(rbind,(lapply(.files, readBin  , double() , 
                        size = 4 ,n =1440 * 720 , signed = T)))
       # take the moving average across the 12 values 
       # from the 12 files for each pixel
       results[[length(results) + 1L]] <- rollapply(x, 3, FUN = mean,na.rm
= T)
   }

But got this error:

       Error in seq.default(start.at, NROW(data), by = by) : 
                                wrong sign in 'by' argument