public class Solution {
    public static int passingCars(int[] a) {
        long passingCars = 0;
        int multiply = 0;

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

        return (int) passingCars;
    }
}