#include <iostream>
#include <math.h>
using namespace std;
typedef double D;
D line(D, D, D, D);
D geron(D, D, D, D, D, D);
int main() {
D x1, y1, x2, y2, x3, y3, x4, y4, s;
cout<<"\nx1= "; cin>>x1; cout<<"y1= "; cin>>y1;
cout<<"\nx2= "; cin>>x2; cout<<"y2= "; cin>>y2;
cout<<"\nx3= "; cin>>x3; cout<<"y3= "; cin>>y3;
cout<<"\nx4= "; cin>>x4; cout<<"y4= "; cin>>y4;
s=geron(x1, y1, x2, y2, x3, y3) + geron(x1, y1, x3, y3, x4, y4);
cout<<"\n Площадь четырёхугольника= "<<s<<endl;
return 0;
}
D line(D a, D b, D c, D d) {
return sqrt((a-c)*(a-c)+(b-d)*(b-d));
}
D geron(D a1, D a2, D b1, D b2, D c1, D c2) {
D p, ab, bc, ca;
ab=line(a1, a2, b1, b2);
bc=line(b1, b2, c1, c2);
ca=line(c1, c2, a1, a2);
p=(ab+bc+ca)/2;
return sqrt(p*(p-ab)*(p-bc)*(p-ca));
}