solve :: String -> Bool
solve = sol 0 0 0 . reverse
    where
        sol :: Int -> Int -> Int -> String -> Bool
        sol _ g r [] = g == 0 && r == 0
        sol w g r (c : cs)
            | c == 'R' = sol w g (r + 1) cs
            | c == 'G' && r == 0 = False
            | c == 'G' = sol w (g + 1) (r - 1) cs
            | c == 'W' && g > 0 = sol (w + 1) (g - 1) r cs
            | c == 'W' && w == 0 = False
            | c == 'W' = sol (w + 1) g r cs
            | otherwise = sol w g r cs

main :: IO ()
main = getLine >> getContents >>= putStr . unlines . fmap (f . solve) . lines
    where
        f True = "possible"
        f _ = "impossible"