#include <iostream>
#include <fstream>
#include <cmath>

using namespace std;

ifstream f ("aria.in");
ofstream g ("aria.out");

int n;
double arie;
struct punct {
    double x, y;
};

double arieTriunghi (punct a, punct b, punct c) {
    return abs ( ((b.x-a.x) * (c.y-a.y) - (c.x-a.x) * (b.y-a.y))/2 );
}

void rezolva () {
    f>>n;
    punct p1, p[4];
    if (n < 3) g<<0;
    else {
        f>>p[1].x>>p[1].y>>p[2].x>>p[2].y>>p[3].x>>p[3].y;
        arie = arieTriunghi (p[1], p[2], p[3]);
        for (int i=4; i<=n; i++) {
            p[2] = p[3];
            f>>p[3].x>>p[3].y;
            arie=arie + arieTriunghi (p[1], p[2], p[3]);
        }

        g<<arie<<'\n';
    }
}

int main () {
    rezolva ();
    return 0;
}