codility - PassingCars (Prefix Sums)

    public int solution(int[] A) {
        // write your code in Java SE 8
 
        int accum = 0;
        int sum = 0;
 
        for (int i = A.length - 1; i >= 0; i--) {
            if (A[i] == 1)
                accum++;
            else
                sum += accum;
                
            if(sum>1000000000) return -1;    
        }
 
        return sum;
 
    }