library(raster)
r <- raster()

userpath <- "C:\somepath"
for(i in 1:203){
NIR <- list.files(path = userpath, pattern = 'NIR,*.*.tif',full.names = T)
RED <- list.files(path = userpath, pattern = 'red*.*.tif', full.names = T)

for (i in 1:203) {
ndvitemp <- overlay(raster(NIR[i]),raster(RED[i]), fun = function(x,y) (x - y)/(x + y))
writeRaster(ndvitemp, paste0(userpath,'NDVI',"_",i,'.tif'))}