UVA-1595 Symmetry

#include <stdio.h>
#include <iostream>
#include <algorithm>
#include <set>

using namespace std;

int main ()
{
    int N;
    cin >> N;
    while(N --)
    {
        int M;
        cin >> M;
        int tmp_M = M;
        set<pair<int,int>> s;
        double m = 0;
        while(tmp_M --)
        {
            int a,b;
            cin >> a >> b;
            m += a;
            s.insert(make_pair(a,b));
        }
    //    cout << m << endl;
        m /= M;
        
        int flag = 1;
        for(auto Ptr = s.begin();Ptr!=s.end();Ptr++)
        {
            int a = 2*m-Ptr->first;
            int b = Ptr->second;
            if(!s.count(make_pair(a,b)))
            {
                flag =  0;
                break;
            }
        }
        if(flag)
            cout << "YES" << endl;
        else
            cout << "NO" << endl;
    }
    return 0;
}