#include<fstream>

using namespace std;
int n, points[100000][2];

ifstream fin("geometry.in");
ofstream fout("geometry.out");

double area(int Ax, int Ay, int Bx, int By, int Cx, int Cy)
{
	return (Ax * (By - Cy) + Bx * (Cy - Ay) + Cx * (Ay - By)) / 2;
}

int main()
{
	fin >> n;

	for (int i = 0; i < n; i++)
		fin >> points[i][0] >> points[i][1];

	double a = 0;
	for (int i = 2; i < n; i++)
	{
		a += area(points[0][0], points[0][1], points[i][0], points[i][1], points[i - 1][0], points[i - 1][1]);
	}

	fout << abs(a);
}