C++ :

// 求出三条边的边长
// 利用海伦公式，求三角形面积
#include <iostream>
#include <cstdio>
#include <cmath>

using namespace std;

int main() {
	double xa, ya, xb, yb, xc, yc;
	scanf("%lf%lf%lf%lf%lf%lf", &xa, &ya, &xb, &yb, &xc, &yc);

	double a = sqrt((xa - xb)*(xa - xb) + (ya - yb)*(ya - yb));
	double b = sqrt((xb - xc)*(xb - xc) + (yb - yc)*(yb - yc));
	double c = sqrt((xa - xc)*(xa - xc) + (ya - yc)*(ya - yc));
	
	double p = (a + b + c) / 2;
	double s = sqrt(p * (p - a) * (p - b) * (p - c));

	printf("%.2lf", s);

	return 0;
}