import qualified Data.ByteString.Char8 as C
import Data.Char

main :: IO ()
main = do
  ss <- C.getLine
  let (x,xs) = nextInt ss
  ss2 <- C.getLine
  let ans = solve 0 x ss2
  print ans
  
solve :: Int -> Int -> C.ByteString -> Int
solve i n ss
  | i == n    = 0
  | otherwise = max d $ solve (i + 1) n xs
  where d      = x - i - 1
        (x,xs) = nextInt ss

nextInt :: C.ByteString -> (Int, C.ByteString)
nextInt ss
  | '0' <= x && x <= '9' = nextInt2 ss 0
  | otherwise            = nextInt $ C.tail ss
  where x = C.head ss

nextInt2 :: C.ByteString -> Int -> (Int, C.ByteString)
nextInt2 ss n
  | C.null ss            = (n, ss)
  | '0' <= y && y <= '9' = nextInt2 ys (n * 10 + (digitToInt y))
  | otherwise            = (n, ys)
  where y  = C.head ss
        ys = C.tail ss