Finding distance between different positions of an object in opencv and c++

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C++

OpenCV

I have written a program to detect a particular object by its color and track it in the webcam frame. My goal is that i want to find the distance of the object from the new point to the previous point. This is my code and i will explain the problem after the code-

x = moment.m10/area;
                  y = moment.m01/area;
                  Point a;
                  a = Point(x,y);
                                    vector<Point> cont(0);
                  cont.push_back(a);
                    for(int i= 0; i < (cont.size()-1); i++)
                  {
                    p = pow((cont.at(i).x - cont.at(i+1).x),2);
                    q = pow((cont.at(i).y - cont.at(i+1).y),2);
                    r = sqrt(p+q);
                    cout<<r<<endl;

                    }

Now, i only get the distance if i have more than one object in the webcam frame and it gives me the distance between object1 and object2 correctly. But what i want is to calculate the distance of a single object from the current position to the new position using the same logic.

Please can anyone help me out. I am really confused as i have no clue where i am doing wrong. Thanks