#include <fstream>
#include <iomanip>
#include <cmath>

using namespace std;
const int NMAX = 100000;

ifstream f("aria.in");
ofstream g("aria.out");

struct punct
{
	long double x, y;
};

int n;
long double A, T;
punct v[NMAX + 1];

int main()
{
	f >> n;
	for(int i = 1; i <= n; i++)
		f >> v[i].x >> v[i].y;
	v[n + 1] = v[1];
	for(int i = 1; i <= n; i++)
	{
		A = v[i].x * v[i + 1].y - v[i].y * v[i + 1].x;
		T += A;
	}
	if(T < 0) T = -T;
	g << fixed << setprecision(5) << T / 2.0;
	f.close();
	g.close();
	return 0;
}