#include<fstream>
#include<utility>
#include<cmath>
#define NMAX 100005
#define x first
#define y second
using namespace std;
int n;
pair<double,double> v[NMAX];
void read()
{
	ifstream fin("aria.in");
	fin>>n;
	for(int i=0;i<n;i++)
		fin>>v[i].x>>v[i].y;
	v[n++]=v[0];
	fin.close();
}
int main()
{
	read();
	double aria=0;
	for(int i=0;i<n;i++)
		aria+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
	aria/=2;
	ofstream fout("aria.out");
	fout.precision(5);
	fout<<fixed<<abs(aria)<<'\n';
	fout.close();
	return 0;
}