#include<fstream>
#include<iomanip>
using namespace std;

double x[100002],y[100002],ar;
int i,n;

int main()
{
	ifstream f("aria.in");
	ofstream g("aria.out");
	f >> n;
	for (i=1;i<=n;i++)
		f >> x[i] >> y[i];
	ar=0;
	x[0]=x[n];y[0]=y[n];
	x[n+1]=x[1];y[n+1]=y[0];
	for (i=1;i<=n;i++)
	{
		ar+=x[i]*y[i+1];
		ar-=x[i]*y[i-1];
	}
	if (ar<0)
		ar*=-1;
	ar/=2;
	g << setprecision(6) << fixed << ar;
	return 0;
}