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

main :: IO ()
main = solve <$> readLn <*> (readil B.readInt <$> B.getLine) >>= print

solve :: Int -> [Int] -> Int
solve n (v:vs) = dp !! n
  where dp = 0 : v : unfoldr f (0, vs)
          where f (_, []) = Nothing
                f (i, (x:xs)) = Just (max (dp !! (i+1)) (x + dp !! i), (i+1, xs))

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