$<.each{|s|
 r=g=n=0;i=0
 puts s.chop.chars.reverse_each{|c|
  case c
  when ?R
    r+=1
  when ?G
    break if 0>r-=1
    g+=1
  else
    (break if n<1)if g<1
    g-=1
    n+=1
  end
 }&&n>0&&r<1&&g<1 ? "possible" : "impossible"
}