#include<iostream>
#include<fstream>
using namespace std;
int main()
{


	double x[100002], y[100002], sumx = 0, sumy = 0, xg, yg, arie = 0;
	long i, n;
	ifstream f("aria.in");
	ofstream g("aria.out");
	f >> n;
	for (i = 1; i <= n; i++)
	{
		f >> x[i] >> y[i];
		sumx += x[i];
		sumy += y[i];

	}

	xg = sumx / n;
	yg = sumy / n;
	for (i = 1; i < n; i++)
	{
		arie += xg*(y[i] - y[i + 1]) + x[i] * (y[i + 1] - yg) + x[i + 1] * (yg - y[i]);
	}
	arie += xg*(y[n] - y[1]) + x[n] * (y[1] - yg) + x[1] * (yg - y[n]);
	arie = arie*0.5;
	g << arie << "\n";
	return 0;

}