#include <vector>

long long passingCars(const std::vector<int>& a)
{
    long long passing = 0;
    long long multiply = 0;

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

    return passing;
}