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

using namespace std;

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

int n;
double s;

struct puncte
{
    double x;
    double y;
};
puncte v[100000];

int main ()
{
    f>>n;
    for(int i=1;i<=n;i++)
    {
        f>>v[i].x;
        f>>v[i].y;
    }
    s=0;
    for(int i=1;i<=n-1;i++)
        s=s+((v[i].y+v[i+1].y)*(v[i+1].x-v[i].x))/2;
    s=s+((v[1].y+v[n].y)*(v[1].x-v[n].x))/2;
    s=abs(s);
    g<<fixed<<setprecision(20)<<s;
    f.close();g.close();
    return 0;
}