Number of Parallelograms

### 题目描述 给定平面上$n$个点，问用这些点能组成多少平行四边形（多个平行四边形可以共用点，保证任意三点不共线） ### 输入格式 第一行输入点数$n$ 随后$n$行输入点的坐标$(x_i,y_i)$，$x$和$y$用空格隔开（见样例） ### 输出格式 输出能组成的四边形数量$c$ ### 数据范围 $1 \leq n \leq 2000$ , $0\leq x_i,y_i \leq 10^{9}$

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 the vertices at the given points.

The first line of the input contains 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}<=10^{9} $ ) — the coordinates of the $ i $ -th point.

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

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