import Data.Array.ST
import Data.Array.Unboxed
import Data.Graph
import Data.ByteString.Char8 qualified as B
import Control.Monad.ST
import Control.Monad

int = (\(Just(x,_)) -> x) . B.readInt

readInput :: [[Int]] -> (Graph, Int, Int)
readInput ([n,m,c]:fs) = (buildG (1,n+m) $ concat $ buildEdges 1 fs, m, c)
 where
  buildEdges _ [] = []
  buildEdges i (_:es:es') = [ [(i,j+m), (j+m,i)] | j <- es ] ++ buildEdges (i+1) es'

solve :: (Graph, Int, Int) -> IO ()
solve (g,m,c)
 | null bands = putStrLn "impossible"
 | otherwise = mapM_ putStrLn ["possible", show $ length bands, unwords $ show <$> bands]
 where
  (1,n) = bounds g
  bands = (\i -> i - m) <$> filter (attends!) [m+1..n]
  minDegree :: UArray Int Int = listArray (1,n) $ [quot (1 + indegree g ! i) 2 | i <- [1..m]] ++ replicate (n - m) c
  attends = runSTUArray $ do
   degree <- (thaw $ outdegree g) :: ST s (STUArray s Int Int)
   attends <- newArray (1,n) True
   let check i = do
        rm <- (&&) <$> readArray attends i <*> ((< (minDegree ! i)) <$> readArray degree i)
        when rm $ do
         writeArray attends i False
         forM_ (g!i) $ \j -> modifyArray' degree j pred >> check j
   forM_ [1..n] check
   return attends

main = B.getContents >>= solve . readInput . map (map int . B.words) . B.lines