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

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

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