sub r{/./?$_=&r:0for$u[$r=$_];$r}
sub u{($a,$b)=map r,@_;@u[$b,$a]=$a}
<>;
$h=3e5;
map{
	++$%,$x*2<$l+$h?
	map{u$x++,$l++}$l..$x-1:
	map{u--$h+$l-$x,$h}$x..$h-1
    if($x=r)-$l;
}glob<>;
print$%;