#include <fstream>
#include <iomanip>
using namespace std;

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

struct point
{
	double x, y;
};

double sf(int n)
{
	long double surface = 0;
	point p1, p2, ps;
	if (n < 3)
		return 0;
	f >> ps.x >> ps.y >> p2.x >> p2.y;
	p1 = ps;
	for (int i = 0; i < n; i++)
	{
		surface += (p1.x * p2.y - p2.x * p1.y);
		p1 = p2;
		f >> p2.x >> p2.y;
	}
	surface += (p2.x * ps.y - ps.x * p2.y);
	return surface / 2;
}

int main()
{
	int n;
	f >> n;
	double x = sf(n);
	long long x1 = (long long)x;
	if (x < 0) x = -x;
	g << x1 << ".";
	x -= x1;
	if (x != 0)
		g << (int)(x * 1000000);
	else g << 0;
	return 0;
}