D.cpp

#include <bits/stdc++.h>
#define int long long
#define FOR(i,a,b) for(int i=(a),_b=(b); i<=_b; ++i)
#define FORD(i,a,b) for(int i=(a),_b=(b); i>=_b; --i)
#define REP(i,a) for(int i=0,_a=(a); i < _a; ++i)

#define DEBUG(X) { cout << #X << " = " << (X) << endl; }
#define PR(A,n)  { cout << #A << " = "; FOR(_,1,n) cout << A[_] << ' '; cout << endl; }
#define PR0(A,n) { cout << #A << " = "; REP(_,n) cout << A[_] << ' '; cout << endl; }

#define sqr(x) ((x) * (x))
#define ll long long
#define __builtin_popcount __builtin_popcountll
#define SZ(x) ((int) (x).size())
using namespace std;

int GI(int& x) {
    return scanf("%lld", &x);
}

struct Point {
    int x, y;
    Point() {}
    Point(int x, int y) : x(x), y(y) {}

    Point operator + (const Point& a) const { return Point(x+a.x, y+a.y); }
} a[2011];
bool operator < (const Point& a, const Point& b) {
    if (a.x != b.x) return a.x < b.x;
    return a.y < b.y;
}
map< Point,int > cnt;

#undef int
int main() {
#define int long long
    ios :: sync_with_stdio(0); cin.tie(0);
    cout << (fixed) << setprecision(9);
    int n;
    while (GI(n) == 1) {
        FOR(i,1,n) {
            GI(a[i].x); GI(a[i].y);
        }
        cnt.clear();
        FOR(i,1,n) FOR(j,i+1,n) cnt[a[i] + a[j]] += 1;

        int res = 0;
        for(auto p : cnt) res += p.second * (p.second - 1) / 2;
        cout << res << endl;
    }
}