function solution($A) {
  $count = count($A);
  $left = 0;
  $right = 0;

  if (!$count) {
    return -1;
  }

  if ($count == 1) {
    return 0;
  }

  for ($n = 1; $n < $count; $n++) {
    $right += $A[$n];
  }

  if ($left == $right) {
      return 0;
  }

  for ($n = 1; $n < $count; $n++) {
      $left += $A[$n - 1];
      $right -= $A[$n];

      if ($left == $right) {
          return $n;
      }
  }

  return -1;
}