#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
{
	double x, y;
} points[NMAX];

int points_size;
double area;

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

void Solve()
{
	
}


int main()
{
	out << setprecision(6) << fixed;
	Read_Data();
	for (int i = 1; i <= points_size; i++)
		area = area + (double)(points[i].x * points[i + 1].y - points[i + 1].x * points[i].y) / 2;
	out << area << "\n";
	return 0;
}