Usage of accumulate with a custom object

I am using opencv library with C++, and I am trying to compute sum of points which are contained in vector<Point2f> difference

Point class has x property which is float.

float pointSumX(Point2f pt1,Point2f pt2)
{
    return (pt1.x + pt2.x);
}

I defined function as above, and call it from accumulate shown below. But it throws error.

float avgMotionX = accumulate(difference.begin(),difference.end(),0,pointSumX);

Error is:

error: could not convert ‘__init’ from ‘int’ to ‘cv::Point_’ __init = __binary_op(__init, *__first);

Note: I am using C++11