#include <fstream>
#include <cmath>
#include <iomanip>

using namespace std;

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

struct point {
    double x, y;
};
double area (int n, point p[]) {
    double area_tot = 0;
    for(int i=1; i<n; i++) {
        area_tot += p[i].x*p[i+1].y-p[i+1].x*p[i].y;
    }
    area_tot += p[n].x * p[1].y - p[1].x*p[n].y;
    return area_tot;
}
int n, i;
point p[100003];

int main()
{
    f>>n;
    for(i=1; i<=n; i++)
        f>>p[i].x>>p[i].y;
    g<<setprecision(6)<<fixed<<area(n, p)/2.0;
    return 0;
}