import Control.Applicative
import Control.Monad
import Control.Monad.ST
import Data.Functor
import Data.Function
import Data.Monoid
import Data.Maybe
import Data.List
import qualified Data.Foldable as Foldable
import Data.List.Split
import Data.Bits
import Data.Char
import Data.Ix
import Data.Ratio
import Data.Ord
import Data.Tuple
import Data.Array
import Data.Array.IO
import Data.Array.ST
import Data.IORef
import Data.STRef
-- import System.IO.Unsafe
 
readInt = read :: String -> Int
readDouble = read :: String -> Double
getInt = readLn :: IO Int
getInts = map readInt . words <$> getLine
getDouble = readLn :: IO Double
sjoin :: (Show a) => [a] -> String
sjoin = unwords . map show
cond :: a -> a -> Bool -> a
cond t f c = if c then t else f
apply2 :: (a -> a -> b) -> [a] -> b
apply2 f [x,y] = f x y
apply3 :: (a -> a -> a -> b) -> [a] -> b
apply3 f [x,y,z] = f x y z
fnBin :: (b -> c -> d) -> (a -> b) -> (a -> c) -> a -> d
fnBin op f g x = op (f x) $ g x
fnTuple :: (a -> b, a -> c) -> a -> (b, c)
fnTuple (f,g) a = (f a, g a)
replace :: (Eq a) => a -> a -> [a] -> [a]
replace x y = map (\z -> if z==x then y else z)
modifyArray :: (Ix i, MArray a e m) => a i e -> i -> (e -> e) -> m ()
modifyArray arr ix f = readArray arr ix >>= writeArray arr ix . f
-- end of templete

seqs xs = map (\xs@(x:_) -> (x, x+length xs)) $ groupBy (\x y -> x+1==y) xs

f (0:xs) = max (1 + f xs) $ maximum $ 0 : [rr-ll | ((ll, lr), (rl, rr)) <- zip ss (tail ss), lr+1==rl]
  where
    ss = seqs xs

f xs = maximum $ map (\(l,r) -> r - l) $ seqs xs


main = do
    [n, k] <- getInts
    when (n/=0 || k/=0) $ do
        a <- replicateM k getInt
        let sorted = sort a
        print $ f sorted
        main