#include <fstream>
#include <iomanip>

#define NMAX 100005

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

int n;
long double answer;

struct Point{
    double x;
    double y;
};
Point v[NMAX];

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

        if(i > 1)
        {
            answer = answer + (v[i - 1].x * v[i].y - v[i].x * v[i - 1].y);
        }
    }
    answer = answer + (v[n].x * v[1].y - v[1].x * v[n].y);

    g << fixed;
    g << setprecision(10) << answer / 2.0 << "\n";
    return 0;
}