#include<fstream>
#include<iomanip>
using namespace std;
int x[100002];
int y[100002];
int main()
{
	int n,i;
	long double aria=0;
	ifstream f("aria.in");
	f>>n;
	for(i=1;i<=n;i++)
		f>>x[i]>>y[i];
	f.close();
	x[n+1]=x[1];
	y[n+1]=y[1];
	for(i=1;i<=n;i++)
		aria=aria+(((long double)y[i+1]+y[i])*(x[i+1]-x[i]));
	if(aria<0)
		aria=-aria;
	aria=(long double)aria/2;
	ofstream g("aria.out");
	g<<fixed<<setprecision(5)<<aria<<'\n';
	g.close();
	return 0;
}