https://codility.com/demo/results/trainingH8J8AX-HMM/

PassingCars.py

# you can write to stdout for debugging purposes, e.g.
# print "this is a debug message"

def solution(A):
    # write your code in Python 2.7
    L = len(A)
    n = 0
    
    if L == 1:
        return 0
        
        
    for i in range(L-2, -1, -1):
        if A[i]:    # 1 or west
            A[i] = A[i+1] + 1
        else:       # 0 or east
            A[i] = A[i+1]
            n = n + A[i]
            if n > 1000000000:
                return -1
                
    return n