[codility] Task name: PassingCars

https://codility.com/c/intro/demoQQDY3P-6DY

int solution(NSMutableArray *A) {
    // write your code in Objective-C 2.0
    
    int east = 0, west = 0;

    for (NSNumber *num in A)
    {
        int dir = [num intValue];
        (dir) ? west++ : east++;
    }
    
     int wc = 0; 
     int ec = 0;
     
     int ret = 0;
     
    for (int i = 0 ; i < A.count; i++)
    {
        
        if ([A[i] intValue])    // 1, west
        {
            ret = ret + ec;
        }
        else // 0, east
        {
            ec++;       
        }
    }

    if (ret > 1000000000) return -1; // if car pair exceeds 1000000000
    else return ret;
}