counting vehicle number with cascade

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hi, I wrote this codes to detect vehicles inside each frame in the video. Now I want to count them but I don't know how to do it. Could you show me how, please?

#include "stdafx.h"
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/objdetect/objdetect.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <iostream>
#include <vector>

using namespace cv;
using namespace std;

int main(){

    VideoCapture cap("arabalar2.avi"); // open the video file for reading

    if ( !cap.isOpened() )  // if not success, exit program
    {
         cout << "Cannot open the video file" << endl;
         return -1;
    }

    Mat gray;

    string cascade_file= "C:/Cascades/haarcascades/cars.xml";
    CascadeClassifier cascade;

    if (cascade_file.empty() || !cascade.load(cascade_file))
    {
        cout << "Hata: cascade dosyası bulunamadı!n";
        return -1;
    }

    namedWindow("MyVideo",CV_WINDOW_NORMAL);  

    while(1){

        Mat frame;

        bool bSuccess = cap.read(frame); // read a new frame from video

         if (!bSuccess) //if not success, break loop
         {
              cout << "Cannot read the frame from video file" << endl;
              break;
         }

        cvtColor(frame, gray, CV_BGR2GRAY);
        equalizeHist(gray, gray);
        vector<Rect> cars;

        cascade.detectMultiScale(gray, cars, 1.1, 3, CV_HAAR_DO_CANNY_PRUNING|CV_HAAR_SCALE_IMAGE, Size(30,30));

        for (int i = 0; i < cars.size(); i++) // draw a rectangle for dedected vehicles
        {
            Rect rect = cars[i];
            rectangle(frame, rect, CV_RGB(0,255,0), 1);
        }

        imshow("MyVideo", frame);

        if(waitKey(30) == 27) //wait for 'esc' key press for 30 ms. If 'esc' key is pressed, break loop
        {
                cout << "esc key is pressed by user" << endl; 
                break; 
        }

    }
    return 0;
}