#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;
} p[100001];

void citeste () {
    f>>n;
    for (int i = 1; i <= n; i++) f>>p[i].x>>p[i].y;
}

void rezolva () {
    p[n+1] = p[1];
    for (int i=1; i<=n; i++) {
        arie += (p[i].x * p[i+1].y - p[i+1].x * p[i].y);
    }
    arie = arie / 2;
    g<<arie<<'\n';
}

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