#include <fstream>
#include <iomanip>

#define NMAX 100005
#define input "aria.in"
#define output "aria.out"

using namespace std;

ifstream in(input);
ofstream out(output);

struct p
{
	long double x, y;
} points[NMAX];

int points_size;
double area;

double Abs(double a)
{
	return a < 0 ? -a : a;
}

void Read_Data()
{
	in >> points_size;
	for (int i = 1; i <= points_size; i++)
		in >> points[i].x >> points[i].y;
}

double Triangle_Area(p A, p B, p C)
{
	return (double)(((B.x - A.x)*(C.y - A.y) - (C.x - A.x)*(B.y - A.y)) / 2);
}

void Solve()
{
	for (int i = 2; i < points_size; i++)
		area = (double)area + Triangle_Area(points[1], points[i], points[i + 1]);
	out << area << "\n";
}


int main()
{
	out << setprecision(6) << fixed;
	Read_Data();
	Solve();
	return 0;
}