import Control.Applicative
import Data.List

main :: IO()
main = do
  n <- readLn :: IO Int
  as <- map read . lines <$> getContents :: IO [Integer]

  let
    x = sum $ map (`div` 2) as
    y = map (`mod` 2) as

  print $ x + (sum . map ((`div` 2) . length) . filter (elem 1) . group) y
