#include <fstream>
using namespace std;

int n,x[1000000],y[1000000];
float s=0;

void lire()
{
	ifstream f("aria.in");
	f >> n;
	for (int i=1; i<=n; i++)
		f >> x[i] >> y[i];
	f.close();
}

void ecrire()
{
	ofstream f("aria.out");
	f << s;
	f.close();
}

int main()
{
	lire();
	for (int i=1; i<n; i++)
		s=s+(x[i]*y[i+1]-x[i+1]*y[i])/2;
	s=s+(x[n]*y[1]-x[1]*y[n])/2;
	ecrire();
	return 0;
}