# $X = missing_n(\@sorted_numbers);

sub missing_n {
  my ($L, $U) = (0, $#{ $_[0] });
  my $p = $L + int(($U - $L)/2);

  return 1 if $_[0][0] != 1;
  return $_[0][-1] + 1 if $_[0][-1] == @{ $_[0] };

  until ($L == $U) {
    if ($_[0][$p] == $p+1) { $L = $p + 1 }
    else { $U = $p }
    $p = $L + int(($U - $L)/2);
  }

  return $_[0][$p-1] + 1;
}