import Data.Map.Strict (Map)
import qualified Data.Map.Strict as Map

getInts = fmap (map read . words) getLine

main = do
  getLine
  a <- getInts
  print $ solve a
  
solve :: [Int] -> Int
solve ns = fn' $ Map.toAscList $ fn ns
  where fn [] = Map.empty
        fn (n:ns) = Map.insertWith (+) n 1 $ fn ns
        fn' [] = 0
        fn' ((k,v):[]) = v
        fn' (n1@(k1,v1):n2@(k2,v2):ns) = max (if (k2-k1) <= 1 then (v1+v2) else v2) (fn' (n2:ns))