import Data.Bool
import Data.List

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 (isClosed w) ims)
 where ims = zip [i | i<-[1..(w+1)*h], mod i (w+1) /= 0] ms

isClosed w [] = Nothing
isClosed w ((i,m):ims) = Just (dfs w [i,i] is, ims)
 where is = map fst (filter ((==m) . snd) ((i,m):ims))

dfs w acc@(a:a':as) is
 | null is = False
 | notElem a is = False
 | elem a as = True
 | otherwise = or (map (\i -> dfs w (i:acc) is) (delete a' [a-1,a+1,a-(w+1),a+(w+1)]))