import Data.List

fork f (x,y) = (f x, f y)

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

pyramid n as = length ((ipos n as) \\ (ppos (sum as)))

ipos n as = [(x,y) | x <- [0 .. n-1], y <- [0 .. (as!!x - 1)]]

ppos x = last $ takeWhile ((<= x) . length) $ iterate shift [(0,0)]
 where shift ps = [(i,0) | i<-[0 .. (maximum (map fst ps) + 2)]] ++ map (fork succ) ps