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

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

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