my $n = 28;
my($a, $b, $d, $x, $y) = (0, 1, 1, 1, 0);
{
        0 != ($n & 1) and ($x, $y) = ($a*$x + $b*$y, $b*$x + $d*$y);
        $n <= 1 and last;
        $n >>= 1;
        ($a, $b, $d) = ($a*$a + $b*$b, $a*$b + $b*$d, $b*$b + $d*$d);
redo}
print $y, "\n";