require(raster)
require(rgdal)
require(sp)
require(snow)
setwd("E:/neeraj_pandey/neeraj_pandey/2016")
test=list.files(getwd(), pattern="tif$", full.names=FALSE)
test
NDVI=function(a,b){
  den=a-b
  num=a+b
  nd=den/num
  nd[is.na(nd)]=-999
  return(nd)
}
for(i in c(1,3,5,7,9,11,13,15,17,19,21,23,25,27)){
  red=raster(test[i])
  nir=raster(test[i+1])
  out=overlay(nir,red,fun=NDVI,progress='text')
  outfilename=paste('NDVI',i,sep = ' ')
  writeRaster(out,outfilename,format= 'ENVI')
}