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

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

struct Punct{
    double x, y;
};

int main()
{
    Punct a[100100];
    int n;
    double x, y, S = 0;
    fin >> n;
    for (int i = 1; i <= n; i++)
        fin >> a[i].x >> a[i].y;
    for (int i = 1; i <= n; i++)
        S += (a[i].x * a[i + 1].y - a[i].y * a[i + 1].x) / 2;
    S += (a[n].x * a[1].y - a[n].y * a[1].x) / 2;
    fout << fixed << setprecision(5) << abs(S) << '\n';

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