import Control.Monad
import Control.Applicative
import Data.List
import Data.Array

main = do
    n <- readLn
    vs <- (map read .words) <$> getLine
    
    print (solve n vs)

maxs x y = if x >= y then x
                    else y

solve n vs
  | n == 1 = vs !! 0
  | n == 2 = maxs (vs !! 0) (vs !! 1)
  | otherwise = a!n
                where a = array (1,n) (  (1,(vs !! 0)) : (2,maxs (vs !! 0) (vs !! 1)) : [(i,maxs (a!(i-1)) ((vs !! (i-1)) + a!(i-2)) ) | i <- [3,4..n]]  )