#include <fstream>
#include <iomanip>

#define NMax 100010

using namespace std;

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

int n;
double surface;
struct mPoint
{
	double x;
	double y;
}points[NMax];

int main()
{
	f >> n;
	for (int i = 1; i <= n; i++)
		f >> points[i].x >> points[i].y;

	for (int i = 1; i <= n - 1; i++)
		surface += points[i].x * points[i + 1].y - points[i + 1].x * points[i].y;
	surface += points[n].x * points[1].y - points[1].x * points[n].y;
	surface /= 2;
	
	g << setprecision(5) << fixed << surface;
	
	return 0;
}