#include<fstream>
#include<iomanip>

using namespace std;

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

struct punct{
	double x,y;
};

int n;
double s=0;
punct a[1000000];

int main()
{
	fin>>n;
	for(int i=1;i<=n;++i)fin>>a[i].x>>a[i].y;
	for(int i=1;i<=n-1;++i)s=s+0.5*(a[i].x*a[i+1].y-a[i+1].x*a[i].y);
	s=s+0.5*(a[n].x*a[1].y-a[n].y*a[1].x);
	fout<<setprecision(5)<<s;
	return 0;
}