#include <bits/stdc++.h>
using namespace std;
using ld = long double;
constexpr int T = 100000;
constexpr ld pi = acos(-1);
int main() {
	cin.tie(0)->sync_with_stdio(0);
	int n;
	cin >> n;
	vector<int> x(n), y(n);
	for (int i = 0; i < n; i++) {
		cin >> x[i] >> y[i];
	}
	vector<ld> z(n);
	ld sum = 0;
	for (int i = 0; i < T; i++) {
		auto t = polar<ld>(1, 2 * pi / T * i);
		ld c = t.real(), s = t.imag();
		for (int j = 0; j < n; j++) {
			z[j] = c * x[j] + s * y[j];
		}
		sort(z.begin(), z.end());
		for (int j = 1; j < n; j++) {
			sum += (z[j] - z[j - 1]) * j * (n - j);
		}
	}
	cout << fixed << setprecision(10) << sum / T * pi / 2 << "\n";
	return 0;
}