CF181B Number of Triplets

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

Print the single number — the answer to the problem.