import Data.Bool
import Data.List

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

bus m = validStart m && validFreq m && validLine m

validStart m = head m == 'W' && elem 'G' (takeWhile (/='R') m)

validFreq m = all (>0) [w,g,r] && g == r && w >= g && w >= r
 where
  w = length (filter (=='W') m)
  g = length (filter (=='G') m)
  r = length (filter (=='R') m)

validLine = all bsucc . takeWhile (not.null) . tails
 where
  bsucc ('W':bs) = elem 'G' bs
  bsucc ('G':bs) = elem 'R' bs
  bsucc _ = True