#include<stdio.h>
#include<fstream>
#include<iomanip>
#include<utility>

using namespace std;

FILE *in;

//definitii 
#define x first
#define y second
#define pdd pair<double,double>

//functii
long double det(pdd a, pdd b, pdd c);

//variabile
ofstream out("aria.out");

int n;
pdd fix,curent,precedent;
long double aria;

int main(void)
{
	in=fopen("aria.in","rt");
	
	fscanf(in,"%d",&n);
	fscanf(in,"%lf%lf",&fix.x,&fix.y);
	fscanf(in,"%lf%lf",&precedent.x,&precedent.y);
	
	for(int i=3 ; i<=n ; ++i)
	{
		fscanf(in,"%lf%lf",&curent.x,&curent.y);
		aria += det(fix,precedent,curent);
		precedent = curent;
	}
	
	out << fixed << setprecision(5) << aria/2.0;
	out << '\n';
	
	fclose(in);
	out.close();
	return 0;
}

long double det(pdd a, pdd b, pdd c)
{
	return a.x * b.y + a.y * c.x + b.x * c.y - b.y * c.x - a.y * b.x - a.x * c.y;
}