{-# LANGUAGE BangPatterns #-}

import           Control.Arrow
import qualified Data.ByteString.Char8             as BSC8
import qualified Data.Vector.Fusion.Stream.Monadic as VFSM
import qualified Data.Vector.Unboxed               as VU
import qualified Data.Vector.Unboxed.Mutable       as VUM

stream :: Monad m => Int -> Int -> VFSM.Stream m Int
stream !l !r = VFSM.Stream step l
  where
    step x
      | x < r     = return $ VFSM.Yield x (x + 1)
      | otherwise = return $ VFSM.Done
    {-# INLINE [0] step #-}
{-# INLINE [1] stream #-}

rep :: Monad m => Int -> (Int -> m ()) -> m ()
rep n = flip VFSM.mapM_ (stream 0 n)
{-# INLINE rep #-}

type Parser a = BSC8.ByteString -> Maybe (a, BSC8.ByteString)
parseInt :: Parser Int
parseInt = fmap (second BSC8.tail) . BSC8.readInt
parseM :: Int -> IO (VU.Vector Int)
parseM m = VU.unfoldrN m parseInt <$> BSC8.getLine

main :: IO ()
main = do
  m  <- readLn :: IO Int
  vs <- parseM m
  dp <- VUM.unsafeNew 1001
  rep m $ \i -> do
    if (i < 2)
      then VUM.unsafeWrite dp i (vs VU.! i)
      else if i == 2
        then VUM.unsafeWrite dp i (vs VU.! 2 + vs VU.! 0)
        else do
          item1 <- VUM.unsafeRead dp (i - 2)
          item2 <- VUM.unsafeRead dp (i - 3)
          let item = max item1 item2
          VUM.unsafeWrite dp i (item + vs VU.! i)
  n1 <- VUM.unsafeRead dp (m - 1)
  n2 <- VUM.unsafeRead dp (m - 2)
  print $ max n1 n2