#include <fstream>
#include <iomanip>

using namespace std;

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

int i,n;
double atot;
struct hh{
    double x,y;
};
hh a,b,a1;
double arie(double x1,double y1,double x2,double y2)
{
    return x1*y2-y1*x2;
}
int main()
{
    f>>n;
    f>>a.x>>a.y;
    a1=a;
    for(i=2;i<=n;i++)
    {
        f>>b.x>>b.y;
        atot+=arie(a.x,a.y,b.x,b.y);
        a=b;
    }
    atot+=arie(b.x,b.y,a1.x,a1.y);
    atot=atot/2;
    g<<fixed<<setprecision(6)<<atot;
}