#include <iomanip>
#include <fstream>
#include <vector>

using namespace std;

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

int main() {
    int N;
    vector<pair<double, double> > points;

    fin >> N;
    for (int i = 0; i < N; i++) {
        double x, y;
        fin >> x >> y;
        points.push_back(make_pair(x, y));
    }


    double sum = 0;
    for (size_t i = 0; i < N; i++) {
        sum += 0.5 * (points[i].first * points[(i+1) % N].second - points[(i+1) % N].first * points[i].second);
    }

    fout << fixed << setprecision(6) << sum << '\n';

    return 0;
}