module Main where

solve :: [Int] -> Maybe Int
solve bs = go bs 0
  where
    go [] acc = Just acc
    go [x] acc = if even x then Just acc else Nothing
    go (x : y : xs) acc
        | even x = go (y : xs) acc
        | otherwise = go (y + 1 : xs) (acc + 2)

main :: IO ()
main = do
    n <- readLn :: IO Int
    bs <- map read . words <$> getLine :: IO [Int]
    case solve bs of
        Just acc -> print acc
        Nothing -> putStrLn "NO"