#include <iostream>
#include <fstream>
#include <iomanip>
#define Nmax 100005

using namespace std;

string file="aria";

ifstream f( (file + ".in").c_str() );
ofstream g( (file + ".out").c_str() );

pair <double, double> v[Nmax];
double x, y;
int n;

double aria()
{
    n++;
    v[n]=v[1];
    double s=0;
    for (int i = 1; i <= n; i++){
        s+=v[i].first*v[i+1].second-v[i].second*v[i+1].first;
    }
    return s/2;
}

int main()
{
    f >> n;

    for (int i = 1; i <= n; i++){
        f >> x >> y;
        v[i]={x, y};
    }

    g << fixed << setprecision(5) << aria();

    return 0;
}