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

using namespace std;

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

struct coordonate{
    double x;
    double y;
};

int n;
double suma;
double arie;
coordonate coord[100005];

int main()
{
    in>>n;
    for(int i=1; i<=n; i++)
        in>>coord[i].x>>coord[i].y;

    coord[n+1].x=coord[1].x;
    coord[n+1].y=coord[1].y;

    for(int i=1; i<=n; i++)
        suma+=(coord[i].x*coord[i+1].y)-(coord[i+1].x*coord[i].y);

    arie=suma/2.0;
    out<<fabs(arie);

    in.close();
    out.close();
    return 0;
}