import Control.Applicative ((<$>))
import Data.ByteString.Char8 (ByteString)
import qualified Data.ByteString.Char8 as B
import Data.Char (isSpace)
import Data.List (foldl', unfoldr)

getL :: (ByteString -> [Int]) -> IO [Int]
getL f = f <$> B.getLine

readIL :: (ByteString -> Maybe (Int, ByteString)) -> (ByteString -> [Int])
readIL f = unfoldr g
    where
        g s = do
            (n, s') <- f s
            return (n, B.dropWhile isSpace s')

main :: IO ()
main = do
    B.getLine
    xs <- getL $ readIL B.readInt
    putStr $ unlines $ map show $ solve $ reverse xs

solve :: [Int] -> [Int]
solve (0:xs)        = solve xs
solve []            = [0, 0]
solve [x]           = [0, x]
solve [x, y]        = [1, y, x]
solve [x, y, z]     = [2, z, y, x]
solve (x:y:z:w:rs)  = zs
    where
        zs = solve (y:z+x:w:rs)