https://keisan.casio.com/exec/system/1223520411
double Area(int x1, int y1, int x2, int y2, int x3, int y3) {
double s = 0.5*abs(x1*y2 + x2*y3 + x3*y1 - y1*x2 - y2*x3 - y3*x1);
cout << "S = " << s << endl;
return s;
}