import Data.Bool
import Data.List

last' s = if null s then "" else last s

main = getLine >> interact (unlines . map (bool "impossible" "possible" . citybus) . lines)

citybus s = and [num, restR, restG, pairGR]
  where
    num = let [g,r,w] = map (pred.length) (group (sort ("WGR"++s))) in g == r && w >= g
    restR = (=="R") $ last' $ filter (\(b:_) -> b=='R') $ delete [] $ tails s
    restG = (notElem 'W') $ last' $ filter (\(b:_) -> b=='G') $ delete [] $ tails s
    pairGR = foldr pairGRgo (Just 0) s == Just 0

pairGRgo _ Nothing = Nothing
pairGRgo 'R' (Just n) = Just (n + 1)
pairGRgo 'G' (Just n)
  | n == 0 = Nothing
  | otherwise = Just (n - 1)
pairGRgo _ m = m