#include<fstream>
#include<cmath>
#define NMAX 100010

using namespace std;

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

struct punct
{
    double x, y;
}poligon[NMAX];

int n;

void Citeste()
{
    int i;

    f>>n;

    for (i=1; i<=n; ++i)
        f>>poligon[i].x>>poligon[i].y;
}

double triunghi(punct A, punct B)
{
    return A.x*B.y-A.y*B.x;
}

double arie()
{
    int i;
    double a=0.0;

    poligon[n+1]=poligon[1];

    for (i=1; i<=n; ++i)
        a+=triunghi(poligon[i], poligon[i+1]);

    return a/2.0;
}

int main()
{
    Citeste();

    g<<fixed<<arie()<<"\n";

    f.close();
    g.close();
    return 0;
}