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

class Triangle{
private:
    double Ax, Ay;
    double Bx, By;
    double Cx, Cy;
    double pl;
public:
    Triangle(){}
    Triangle(double x1, double x2, double x3, double y1, double y2, double y3){
        Ax = x1;
        Bx = x2;
        Cx = x3;
        Ay = y1;
        By = y2;
        Cy = y3;
    }
    void setPointA(double x1, double y1){ Ax = x1; Ay = y1; }
    void setPointB(double x1, double y1){ Bx = x1; By = y1; }
    void setPointC(double x1, double y1){ Cx = x1; Cy = y1; }

    double getArea(){
        double ppl =abs(1.0/2   * ((Ax - Cx)*(By - Cy) - (Bx - Cx)*(Ay - Cy)));
        this->pl = ppl;
        return ppl;
    }

};

int main(){
    setlocale(LC_ALL, "Russian");
    Triangle A;
    A.setPointA(1,1);
    A.setPointB(4,5);
    A.setPointC(9,3);
    cout << A.getArea();

}