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

using namespace std;

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


struct Punct{
    double x,y;
};

Punct p[1000004];

double Aria(int n,Punct p[])
{
    p[n+1]=p[1];
    double s=0;
    for(int i=1;i<=n;++i)
    {
        s+=p[i].x * p[i+1].y - p[i].y * p[i+1].x;

    }
    return fabs(s/2.0);
}

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

    fout<<fixed<<setprecision(5)<<Aria(n,p);


    fin.close();
    fout.close();
    return 0;
}