function passingCars(array $a): int
{
    $passingCars = $multiply = 0;
    foreach ($a as $i) {
        if ($i === 0) {
            $multiply++;
        } elseif ($multiply > 0) {
            $passingCars += $multiply;
            if ($passingCars > 1000000000) {
                return -1;
            }
        }
    }

    return $passingCars;
}