How do I draw a bounding box around a person

#include"opencv2\imgproc.hpp"
#include"opencv2\highgui.hpp"
#include"opencv2\video.hpp"
#include"opencv\cv.h"
#include<vector>
#include<stdint.h>
#include<iostream>

using namespace cv;
using namespace std;

Mat currentframe;
Mat prevframe, difframe;
vector<vector<point> > contours;
vector<vec4i> hierarchy;
int area = 0;
int idx = 5;
Rect rect;

int main(int argc, char* argv[]) {

	//Capture Video
	VideoCapture cap("1.mp4");
	if (!cap.isOpened()) {
		cerr << "Unable to open file" << endl;
		return -1;
	}

	

	//namedWindow("Input_Seq", 1);

	namedWindow("Frame Difference", 1);


	while (1) {
		cap >> currentframe;

		if (currentframe.empty())
			break;
		cvtColor(currentframe, currentframe, CV_BGR2GRAY);

		cap >> prevframe;

		if (prevframe.empty())
			break;
		cvtColor(prevframe, prevframe, CV_BGR2GRAY);

		absdiff(currentframe, prevframe, difframe);
		

		//GaussianBlur(difframe, difframe, Size(3, 3), 0, 1, BORDER_DEFAULT);
		Canny(difframe, difframe, 30, 100, 3);
		findContours(difframe, contours, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));

		
		

		imshow("Input_Seq", currentframe);
		imshow("Frame Difference", difframe);
		char c = (char)waitKey(25);
		if (c == 27)
			break;
	}
	cap.release();
	destroyAllWindows();
	return 0;
}