void task4() {
    int n;
    cin >> n;
    vector <int> x_coordinates;
    vector <int> y_coordinates;
    for (int i = 0; i < n; i++) {
        int x, y;
        cin >> x >> y;
        x_coordinates.push_back(x);
        y_coordinates.push_back(y);
    }
    int first_area = 0, second_area = 0;
    double area;
    x_coordinates.push_back(x_coordinates[0]);
    y_coordinates.push_back(y_coordinates[0]);
    for (int i = 1; i < y_coordinates.size(); i++) {
        first_area += x_coordinates[i - 1] * y_coordinates[i];
    }
    for (int i = 1; i < x_coordinates.size(); i++) {
        second_area += x_coordinates[i] * y_coordinates[i - 1];
    }
    area = (first_area - second_area) / 2;
    cout << fixed << setprecision(3) << area;
}