#include <fstream>
#include <iomanip>
using namespace std;
ifstream fi("aria.in");
ofstream fo("aria.out");
double X[100001],Y[100001];
double arie;
int n;
int main()
{
	fi>>n;
	for (int i=1;i<=n;i++)
	{
		fi>>X[i]>>Y[i];
		X[i]+=1000001;
		Y[i]+=1000001;
	}
	X[n+1]=X[1];
	Y[n+1]=Y[1];
	arie=0;
	for (int i=1;i<=n;i++)
		arie=arie+(X[i+1]-X[i])*(Y[i]+Y[i+1]);
	if (arie<0)
		arie=-arie;
		fo<<setprecision(10)<<arie/2;
	fi.close();
	fo.close();
	return 0;
}