import Data.Bool

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

bus m = all (all (>=0)) wgs && last (last wgs) == 0
 where
  wgs = scanl asum [0,0] m
  asum [w,g] 'W' = [w+1, g]
  asum [w,g] 'G' = [w-1, g+1]
  asum [w,g] 'R' = [w, g-1]