/**
 *    author:  longvu
 *    created: 12/07/24 21:37:23
**/
#include<bits/stdc++.h>

using namespace std;

#define int long long
#define ld long double
#define sz(x) ((int)x.size())
#define all(x) (x).begin(), (x).end()
const int INF = numeric_limits<int>::max();
const int nax = (int)(201001);
const int mod = 1e9 + 7;

template<class X, class Y>
bool maximize(X& x, const Y y) {
	if (y > x) {x = y; return true;}
	return false;
}
template<class X, class Y>
bool minimize(X& x, const Y y) {
	if (y < x) {x = y; return true;}
	return false;
}

struct Point {
	int x, y;
};

Point a[nax];
int sufx[nax], sufy[nax], suf[nax];
int32_t main() {
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	int n;
	cin >> n;
	for (int i = 1; i <= n; ++i) {
		int x, y;
		cin >> x >> y;
		a[i] = {x, y};
	}
	auto cal = [&](Point z, Point t) {
		return z.x * t.y - z.y * t.x;
	};
	int sum = 0, total = 0;
	for (int i = 1; i < n; ++i) {
		total += cal(a[i], a[i + 1]);
	}
	total += cal(a[n], a[1]);
	total = abs(total);
	int sumjx = 0, sumjy = 0, sumex = 0, sumey = 0;
	for (int i = 1; i <= n; ++i) {
		sumjx += a[i].x * (n - i);
	}
	for (int i = 1; i <= n; ++i) {
		sumjy += a[i].y * (n - i);
	}
	auto calh = [&](int z) {
		return z * (z + 1) / 2;
	};
	for (int i = 3; i <= n; ++i) {
		sumex += a[i].x * (i - 2);
	}
	for (int i = 3; i <= n; ++i) {
		sumey += a[i].y * (i - 2);
	}
	for (int i = n; i >= 1; --i) {
		sufx[i] = a[i].x + sufx[i + 1];
		sufy[i] = a[i].y + sufy[i + 1];
	}
	int sump = 0;
	for (int j = 2; j <= n; ++j) {
		suf[j] += a[j].x * sufy[j + 1];
		suf[j] -= a[j].y * sufx[j + 1];
	}
	for (int j = n; j >= 1; --j) {
		suf[j] += suf[j + 1];
	}
	for (int i = 1; i < n; ++i) {
		sumjx -= a[i].x * (n - i);
		sumjy -= a[i].y * (n - i);
		sum += a[i].x * sumjy;
		sum -= a[i].y * sumjx;
		sum += suf[i + 1];
		sum += sumex * a[i].y;
		sum -= sumey * a[i].x;
		sumex -= sufx[i + 2];
		sumey -= sufy[i + 2];
	}
	// cout << sum << " " << total << '\n';
	cout << setprecision(21) << fixed << (ld)1.0 * sum / total << '\n';
	return 0;
}