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

ifstream is("aria.in");
ofstream os("aria.out");

struct Punct{
    double x, y;
};

int n;
double s;
vector<Punct> v;

int main()
{
    is >> n;
    v.resize(n + 1);
    for ( int i = 0; i < n; ++i )
        is >> v[i].x >> v[i].y;
    v[n] = v[0];
    for ( int i = 0; i < n; ++i )
        s += v[i].x * v[i + 1].y - v[i].y * v[i + 1].x;
    os << abs(s / 2.0);
    is.close();
    os.close();
    return 0;
}