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

class Polygone
{
private:
    vector<Point2D> v;
public:
    explicit Polygone(const std::vector<Point2D>& points) {
        v = points;
    }

    double area() const {
        double s = 0;
        for (unsigned int i = 1; i < v.size() - 1; ++i) {
            s += abs((v[i].x() - v[0].x()) * (v[i + 1].y() - v[0].y()) - (v[i].y() - v[0].y()) * (v[i + 1].x() - v[0].x()));
        }
        return s / 2;
    }

    unsigned int size() const {
        return v.size();
    }

    Point2D vertex(unsigned int N) const {
        return v[N];
    }
};