#include <fstream>
using namespace std;
#define Nmax 100000
ifstream f("aria.in");
ofstream g("aria.out");

struct Punct
{
	float x, y;
};

int main()
{
	double ariePoligon = 0;
	int n, i;
	Punct P[Nmax];
	f >> n;
	for (i = 0;i < n;i++)
	{
		f >> P[i].x >> P[i].y;
		if (i >= 1)
			ariePoligon += (P[i - 1].x*P[i].y - P[i].x*P[i - 1].y) / 2;
	}
	ariePoligon += (P[n-1].x*P[0].y - P[0].x*P[n-1].y) / 2;
	g << ariePoligon;
	return 0;
}