#include <fstream>
#include <vector>

using namespace std;

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

struct point{
    double x,y;
};

vector <point> a;
double s=0;
int n;

double det(point a,point b)
{
    return a.x*b.y-a.y*b.x;
}

main()
{
    cin >> n;
    a.resize(n+1);
    for (int i=0; i<n; i++)
    {
        cin >> a[i].x >> a[i].y;
    }
    a[a.size()-1]=a[0];
    for (int i=0; i<a.size(); i++)
    {
        s+=det(a[i],a[i+1]);
    }
    cout << s/2 << '\n';
}