#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

struct puncte
{
    double x;
    double y;
};

puncte p[100001];

int i,n;
double arie;

int main()
{
    f>>n;
    for (i=0;i<=n-1;i++)
    {
        f>>p[i].x;
        f>>p[i].y;
    }
    for (i=0;i<=n-1;i++)
    {
        arie=arie+(p[i].y+p[(i+1)%n].y)*(p[i].x-p[(i+1)%n].x)/2;
    }
    arie=abs(arie);
    g<<setprecision(20)<<arie;
    f.close();
    g.close();
    return 0;
}