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