#include <cmath>
#include <iomanip>
#include <fstream>
using namespace std;

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



struct Punct{
    double x, y;
};
Punct p[100004];

double Arie(int n, Punct p[]);

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

    fout << fixed << setprecision(5)<< Arie(n, p);
    fin.close();
    fout.close();
    return 0;
}


double Arie(int n, Punct p[])
{
    p[n+1] = p[1];
    int i;
    double s = 0;
    for ( i = 1; i <= n; ++i)
        s+= p[i].x * p[i+1].y - p[i].y * p[i+1].x;
    return fabs(s/2) ;
}