gets
$<.map{|s|
s=s.chomp.reverse
c=0
while s=~/^R+G.*W/
c+=1;s.sub!(/W+R/,?R)||s[?W]='';s[?R]=s[?G]=''
end
puts c>0&&(s.size<1||s=~/^W+$/)?:possible:'impossible'
}