Count the number of passing cars on the road.
A non-empty array $A$ consisting of $N$ integers is given. The consecutive elements of array $A$ represent consecutive cars on a road.
Array A contains only 0s and/or 1s:

• $0$ represents a car traveling east,
• $1$ represents a car traveling west.

The goal is to count passing cars. We say that a pair of cars $(P, Q)$, where $0 ≤ P < Q < N$, is passing when $P$ is traveling to the east and $Q$ is traveling to the west.
For example, consider array $A$ such that:

A[0] = 0 
A[1] = 1 
A[2] = 0 
A[3] = 1 
A[4] = 1

We have five pairs of passing cars: $(0, 1), (0, 3), (0, 4), (2, 3), (2, 4)$.

Write an efficient algorithm for the following assumptions: 
$N$ is an integer within the range $[1..100,000]$; 
each element of array $A$ is an integer that can have one of the following values: $0, 1$.

Returns the number of pairs of passing cars. 
The function should return −1 if the number of pairs of passing cars exceeds 1,000,000,000.