import Data.Bool
import Data.List
import qualified Data.IntMap as M
import qualified Data.IntSet as S

unionMap f s = S.foldr (\e acc -> S.union acc (f e)) S.empty s

main = do
 [w,h] <- map read . words <$> getLine
 ms <- map read . words <$> getContents :: IO [Int]
 putStrLn (bool "impossible" "possible" (anyClosed w h ms))

anyClosed w h ms = or (unfoldr (dfs w) ims)
 where ims = M.fromDistinctAscList (zip [i | i<-[1..(w+1)*h], mod i (w+1) /= 0] ms)

dfs w ims
 | M.null ims = Nothing
 | otherwise = Just (isClosed w [i,i] (M.keysSet ims'), M.difference ims ims')
 where
  (i,m) = head (M.toList ims)
  ims' = reachable w m ims (S.singleton i) (S.singleton i)

reachable w m ims acc b
 | S.null b = M.filterWithKey (\k _ -> S.member k acc) ims
 | otherwise = reachable w m ims (S.union acc b') (S.difference b' acc)
  where
   b' = unionMap (\i -> S.fromList (filter movable [i-1,i+1,i-(w+1),i+(w+1)])) b
   movable k = M.findWithDefault 0 k ims == m

isClosed w acc@(a:a':as) is
 | S.null is = False
 | not (S.member a is) = False
 | elem a as = True
 | otherwise = or (map (\i -> isClosed w (i:acc) is) (delete a' [a-1,a+1,a-(w+1),a+(w+1)]))