请问POJ1971题

请教POJ1971题
http://acm.pku.edu.cn/JudgeOnline/problem?id=1971
我是求出各条线段的中点，然后将中点进行排序，然后计算平行四边形的个数，但结果是WA，不知为什么

C/C++ code

#include <iostream>#include <vector>#include <algorithm>#include <fstream>using namespace std;struct Point{    float x,y;};Point pt[1001];vector<Point> point;int test,n;bool cmp(Point p1,Point p2){    if(p1.x != p2.x)        return p1.x < p2.x;    return p1.y < p2.y;}int main(){    fstream file;    file.open("1.txt",ios::in | ios::out);    file>>test;    while(test--)    {        file>>n;        point.clear();        int i = 0;        int j = 0;        for(i = 0; i < n; i++)        {            file>>pt[i].x>>pt[i].y;        }        for(i = 0; i < n; i++)        {            for(j = i + 1; j < n; j++)            {                Point temp;                temp.x = (pt[i].x + pt[j].x) / 2;                temp.y = (pt[i].y + pt[j].y) / 2;                point.push_back(temp);            }        }        sort(point.begin(),point.end(),cmp);        vector<Point>::iterator iter;        int ans = 0;        int res = 0;        for(iter = point.begin(); iter != point.end() - 1; iter++)        {            if(iter->x == (iter + 1)->x && iter->y == (iter + 1)->y)                ans++;            else            {                res += ans * (ans + 1) / 2;                ans = 0;            }        }        cout<<res<<endl;    }    system("pause");    return 0;}