#include <iostream>
#include <string>
#include <vector>

using namespace std;

class Point {
public:
    double x{};
    double y{};
private:

};

class Polygon {
public:
    void addPoint(Point input) {
        Points.push_back(input);
    }
    Point getPoint(int index) const {
        if (index >= 0) {
            return Points[index];
        }
    }
    size_t size() const {
        return Points.size();
    }
    double area() const {
        double area{};
        double avg{};
        double dif{};
        int i{};
        while (i < Points.size()/2){ //for each pair of points, iterate over and add appropriate amount to the area (so if there are 6 points this loop will run 3 times)
            avg = (Points[i].y + Points[i+1].y)/2;
            dif = Points[i+1].x-Points[i].x;
            area = area + avg*dif;
            i++;
        }
        avg = (Points[Points.size()-1].y + Points[0].y)/2; //pair the first and last points and repeat the process one final time
        dif = Points[Points.size()-1].x - Points[0].x;
        area = area + avg*dif;
        return area;

    }
    void printPoints()const {
        for (Point i : Points ) {
            cout << "x = " << i.x << " y = " << i.y << endl;
        }
    }
private:
    vector<Point> Points{};
};


int main () {
    Polygon porygon{};
    int numInputs{};
    double pointX{};
    double pointY{};
    Point input{};
    cin >> numInputs;
    int i{};
    while (i < numInputs) {
        cin >> input.x;
        cin >> input.y;
        porygon.addPoint(input);
        i++;
    }
    //porygon.printPoints();
    cout << porygon.area();
}