sub comp {
    my ($f, $s) = @_;
    return -1 if length ($f) != length ($s);

    local $_ = $f ^ $s;
    return -1 unless tr/\x00/1/c == 2;

    my $i = index $_ => "11";
    return -1 unless $i >= 0;
    return -1 unless substr($f, $i, 2) eq reverse substr($s, $i, 2);

    return $i;
}