B. Number of Triplets
binary search; brute force
You are given n points on a plane. All points are different.Find the number of different groups of three points (A, B, C) such that point B is the middle of segment AC. The groups of three points are considered unordered, that is, if point B is the middle of segment AC, then groups (A, B, C) and (C, B, A) are considered the same.
The first line contains a single integer n (3 ≤ n ≤ 3000) — the number of points. Next n lines contain the points. The i-th line contains coordinates of the i-th point: two space-separated integers xi, yi ( - 1000 ≤ xi, yi ≤ 1000).It is guaranteed that all given points are different.
Print the single number — the answer to the problem.