import Control.Applicative
import Control.Monad
import Data.List
import Debug.Trace

check :: String -> String
check str = if check_sub 0 0 0 str then "possible" else "impossible"
  where
    check_sub r g b "" = r == 0 && g == 0
    check_sub r g w (x:xs)
      | x == 'R' = check_sub (r+1) g w xs
      | x == 'G' = if r > 0
          then check_sub (r-1) (g+1) w xs
          else False
      | x == 'W' && g > 0 = check_sub r (g-1) (w+1) xs
      | x == 'W' && w > 0 = check_sub r g (w+1) xs
      | otherwise         = False

main = do
  mapM_ putStrLn =<< map (check.reverse) <$> drop 1 <$> lines <$> getContents