sub r{($x)=@_;$x-$p[$x]?r($p[$x]):$x}sub f{$a[$_]==$a[$_+$d]&&($p[$u=r$_]=r$_+$d)==$u}$/=<>;@a=glob<>;print!grep(($_+1)%$/&&f($d=1)||f($d=$/),@i=@p=0..$#a)&&im,possible