#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

int main()
{
    int N;
    fin >> N;

    struct {
        double x, y;
    } coord[100002];

    for (int i = 1; i <= N; i++)
        fin >> coord[i].x >> coord[i].y;
    coord[N+1] = coord[1];

    double Aria = 0;
    for(int i = 1; i <= N; i++)
        Aria += coord[i].x * coord[i+1].y - coord[i+1].x * coord[i].y;

    Aria *= 0.5;

    fout << setprecision(5) << Aria;

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