import Control.Applicative
import Control.Monad
import Data.List

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

main = do
  mapM_ putStrLn =<< map check <$> drop 1 <$> lines <$> getContents