#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct elem
{
    long double x, y;
};
elem v[100009];
long double arie;
int main ()
{
    int n;
    f >> n;
    for (int i=1; i<=n; i++)
        f >> v[i].x >> v[i].y;
    v[n+1]=v[1];
    for (int i=1; i<=n; i++)
    {
        arie+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    }
    g << fixed << setprecision(6) << arie/2;
}