#
# Usage:
#   factorial($n) = 1*2*...*$n
#   factorial($m, $n) = $m*($m+1)*...*$n
#
sub factorial {
  #divide and conquer
  unshift @_, 1 if 2 != @_;
  my ($m, $n) = @_;

  if ($m < $n) {
    my $k = int($m/2 + $n/2);
    return factorial($m, $k) * factorial($k+1, $n);
  }
  else {
    return Math::BigInt->new($m);
  }
}