my $inversions;
do {
  $inversions = 0;
  @board = (0, shuffle(1..15));
  # @board = (1,0,2..15); # for testing
  for my $x(1..15) {
    for my $y($x+1..15) {
      ++$inversions if $board[$x]>$board[$y]
    }
  }
} while $inversions % 2;