Passing Cars

Python Passing Cars

def passing_cars(a: list[int]) -> int:
    passing = 0
    multiply = 0
    for i in a:
        if i == 0:
            multiply += 1
        elif multiply > 0:
            passing += multiply
            if passing > 1000000000:
                return -1

    return passing

This counts eastbound cars as it scans, then adds them whenever a westbound car appears.