negative value of cv::Mat

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I observe that when the Mat data,which were loaded from a picture, transform negative value to zero after subtracting by a number.But I don't need this transform,what should i do ?pleace. The codes are as follows.

#include<iostream>
#include<opencv2/core/core.hpp>
#include<opencv2/highgui/highgui.hpp>
#include<opencv2/imgproc/imgproc.hpp>
using namespace std;
using namespace cv;
int main()
{

        Mat M;
    Point2i p;
    M = imread("D:\\Documents\\Desktop\\Face data\\AR1\\001\\AR001-1.tif", 1);
    cvtColor(M, M, CV_BGR2GRAY);
    M = M - 300;
    cout << M << endl;

      waitKey(0);
    system("pause");
}