import Control.Monad.ST
import qualified Data.Vector as V
import qualified Data.Vector.Mutable as MV

solve :: [Int] -> Int
solve xs = let v' = V.map (`divMod` 2) v
            in sum (V.map fst v') + sum (V.map snd v') `div` 4 
  where
    v :: V.Vector Int
    v = runST $ do
        v <- MV.replicate 10 0
        mapM_ (\i -> MV.modify v succ (i-1)) xs
        V.freeze v

main :: IO ()
main = do
    n <- readLn :: IO Int
    xs <- concatMap (map read . words) . lines <$> getContents :: IO [Int]
    print $ solve xs