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

using namespace std;

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

struct trapez
{
    double x, y;
};
trapez v[100004];
int main()
{
    int n;
    fin >> n;
    for (int i= 1; i<=n; i++)
    {
        fin >> v[i]. y >> v[i]. x;
    }
    v[n + 1] = v[1];
    double s = 0;
    for (int i = 1; i<=n; i++)
    {
        s = s + v[i].x * v[i + 1].y - v[i + 1].x * v[i].y;
    }
    fout << fixed << setprecision(5) <<  fabs (s/2);
    return 0;
}