function passingCars(a: number[]): number {
  let passingCars = 0;
  let multiply = 0;

  for (const i of a) {
    if (i === 0) {
      multiply++;
    } else if (multiply > 0) {
      passingCars += multiply;
      if (passingCars > 1000000000) {
        return -1;
      }
    }
  }

  return passingCars;
}