PassingCars.java

class Solution {
    public int solution(int[] A) {
        int pairs = 0; 
        int easts = 0;
        for (int i : A) {
            if (i == 0) {
                easts++;    
            } else {
                pairs += easts;
            }
            if (pairs > 1e9) {
                pairs = -1;
                break;
            }
        }
        return pairs;
    }
}