library(raster)
library(rgdal)
NDVI< list.files("C:\Users\path\Desktop\PET\sample\", pattern="*.tif", all.files=F, full.names = FALSE, recursive = TRUE, ignore.case = FALSE)
for (i in 1:length(NDVI))
NDVI <- raster(function(x,y)
(x="NIR"+str(i)+".tif"), (y="red"+str(i)+".tif"), (x-y/x+y)
writeRaster(NDVI,"NDVI", format="GTiff")