#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

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

double A;
double x1, x2, y1, y2;
double xi, yi;

void adaug (double x1, double y1, double x2, double y2)
{
    A+=x1*y2;
    A-=x2*y1;
}

void calculez ()
{
    int n;
    f>>n;
    f>>x1>>y1;
    xi=x1; yi=y1;
    for (int i=1; i<n; i++)
    {
        f>>x2>>y2;
        adaug (x1, y1, x2, y2);
        x1=x2;
        y1=y2;
    }
    adaug (x1, y1, xi, yi);
}


void afisez ()
{
    if (A<0) A=(-1)*A;
    A=A/2;
    g<<fixed<<setprecision(6)<<A;
}

int main ()
{
    calculez();
    afisez ();
    //cout<<1;

    return 0;
}