CF660D.Number of Parallelograms

You are given n points on a plane. All the points are distinct, and no three of them lie on the same line. Find the number of parallelograms with vertices at the given points.

给你平面上的 $n$ 个点。所有点互不相同，且任意三点不共线。求以给定点为顶点的平行四边形的个数。

The first line of the input contains an integer n (1 ≤ n ≤ 2000) — the number of points.

Each of the next n lines contains two integers (x__i, y__i) (0 ≤ x__i, y__i ≤ 109) — the coordinates of the i-th point.

输入的第一行包含一个整数 $n$（$1 \leq n \leq 2000$）—— 点的数量。

接下来的 $n$ 行，每行包含两个整数 $(x_i, y_i)$（$0 \leq x_i, y_i \leq 10^9$）—— 第 $i$ 个点的坐标。

Print the only integer c — the number of parallelograms with vertices at the given points.

输出唯一的整数 $c$ —— 以给定点为顶点的平行四边形的个数。