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

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

    my $r = index $_, "11";

   (substr ($f, $r, 2) eq reverse substr ($s, $r, 2)) ? $r : -1;
}