#include<fstream>
#include<iomanip>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
 
struct punct{
    double x, y;
};
 
const int nmax = 100006;
int n;
double rasp;
punct v[nmax];
 
double modul(double x)
{
    if(x<0)
        return -x;
    else
        return x;
}
 
int main(){
    int player_unu=0;
 
    in>>n;
    for(int i = 1; i<=n; i++)
        in>>v[i].x>>v[i].y;

	v[n+1] = v[1];
    for(int i=1;i<=n;i++)
		rasp += v[i + 1].x * v[i].y - v[i].x * v[i + 1].y;
 
    out<<setprecision(5)<<fixed;
	out<<modul(rasp) / 2<<'\n';
 
    return player_unu;
}