Educational Codeforces Round 11 D. 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 the vertices at the given points.

Input
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
(xi, yi) (0 ≤ xi, yi ≤ 109)
— the coordinates of the i-th point.

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

Example

Input

4
0 1
1 0
1 1
2 0

Output

1

Codeforces (c) C

题意：给你一堆点，问能组成多少个平行四边形。

分析：中点相等的两条不共线线段可以组成平行四边形。

#include <cstdio>
#include <iostream>
#include <utility>
#include <map>
using namespace std;
struct thing
{
int x,y;
} p[2005];
map <pair<int,int>,int> f;
int n,ans;
int main()
{
cin.sync_with_stdio(false);
cin>>n;
for(int i = 1;i <= n;i++) cin>>p[i].x>>p[i].y;
for(int i = 1;i <= n-1;i++)
for(int j = i+1;j <= n;j++)
{
ans += f[make_pair(p[i].x+p[j].x,p[i].y+p[j].y)];
f[make_pair(p[i].x+p[j].x,p[i].y+p[j].y)]++;
}
cout<<ans<<endl;
}