sub mark_outside($)
{
 my $map=shift(@_);
 my $end=1;
 my $rotate=0;
 $$map[0][0]=-2;
 while($end)
 {
  $end=0;
  $rotate++;
  $rotate=1 if($rotate==5);
  for my $x (0..$#$map)
  {
   $x=$#$map-$x if($rotate==3 or $rotate==4);
   for my $y (0..$#{$$map[$x]})
   {
    $y=$#{$$map[$x]}-$y if($rotate==2 or $rotate==3);
    if($$map[$x][$y]==-1)
    {
     if (
       ( $x-1>=0 and $$map[$x-1][$y]==-2 ) or 
       ( $y-1>=0 and $$map[$x][$y-1]==-2 ) or 
       ( $x+1<@{$map} and $$map[$x+1][$y]==-2 ) or 
       ( $y+1<@{$$map[$x]} and $$map[$x][$y+1]==-2 )
      ) 
     {
      $$map[$x][$y]=-2;
      $end=1;
     }
    }
   }
  }  
 }
}