Problems with calculating distance between two points in c++?

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C++

OpenCV

I am really having this irritating problem. I have written a program where a object is detected and tracked in real time. Now, i can see the coordinates of the my tracked object and my goal is to calculate the distance between the new position of the tracker object from the previous one. This is what i did. Say, x and y are the positions of the tracked object. I have pushed back x and y into a point vector and then my goal is to run a loop to calculate the distance. But everytime i run the program it shows, vector subscript out of range. How do i fix it?

int x,y // the position of the tracked object;
vector<Point>cont(0);
Point a;
a = Point(x,y);
cont.push_back(a);
int p,q,r;


for(int i= 0; i <cont.size(); i++)
{
  p = pow((cont.at(i+1).x - cont.at(i).x),2));
  q = pow((cont.at(i+1).y - cont.at(i).y),2));
  r = sqrt(p+q);
 
cout<<"The distance is:"<<r<<endl;
}

}