Miku and lines

Miku likes geometry. While solving few geometrical problems she got stuck on one, luckily for her Kiku came to rescue and solved her problem. Can you solve it too? Given a line $ y =m x + c $, and a set of $N$ points on Cartesian plane in the form of (Xi , Yi) for each $ 1 \leq i \leq N $. You have to find out the number of pair of points which upon joining will make a line segment that will inte