#include <fstream>
#include <math.h>
#include <vector>
#include <set>
#include <algorithm>
#include <cstring>
//#include <unordered_map>
#include <iomanip>
#include <time.h>
#include <stdio.h>
#include <bitset>
#include <map>
#define MAX 500000000000
//#include <iostream>
//#include <windows.h>
#include <deque>
#include <Tlhelp32.h>
using namespace std;
//ifstream cin("jocul.in");
//ofstream cout("jocul.out");
ifstream cin("aria.in");
ofstream cout("aria.out");
pair<float, float> x[100005];
int main()
{
    int n, a, b;
    float ca = 0, cb = 0, s = 0;
    cin >> n;
    for(int i = 0; i < n; i++)
    {
        cin >> a >> b;
        x[i].first = a;
        x[i].second = b;
        ca += a;
        cb += b;
    }
    ca /= float(n);
    cb /= float(n);
    for(int i = 0; i < n; i++)
    {
        if(i < n - 1)
            s += x[i].first * x[i + 1].second + x[i].second * ca + x[i + 1].first * cb - (ca * x[i + 1].second + x[i + 1].first * x[i].second + cb * x[i].first);
        else
            s += x[i].first * x[0].second + x[i].second * ca + x[0].first * cb - (ca * x[0].second + x[0].first * x[i].second + cb * x[i].first);
    }
    cout << s / 2;
}