//* Purposes of using functions *
#include <stdio.h>
#include <math.h>

double sqr (double x);
double DIST(double x1, double y1, double x2, double y2);
double AREA(double a, double b, double c);

int main() {
    double x1, y1, x2, y2, x3, y3;
    double d1, d2, d3, area;

    printf("Please enter the coordinates of the triangle vertices and press ENTER.\n");
    scanf("%lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3);
    d1=DIST(x1, y1, x2, y2);
    d2=DIST(x3, y3, x2, y2);
    d3=DIST(x1, y1, x3, y3);
    area=AREA(d1, d2, d3);
    printf("The area of the triangle is %lf\n",  area);
    return(0);
}

double sqr (double x) {
    return(x*x);
}

double DIST(double x1, double y1, double x2, double y2) {
    return( sqrt( sqr(x2-x1) + sqr(y2-y1) ) );
    // OR
    // return( sqrt( pow( fabs(x2-x1),2)+pow( fabs(y2-y1),2) ) );
    // warning: C library requires that the base in pow is >0 !!!
}

double AREA(double a, double b, double c) {
    // Hero's formula for the triangle area
    double p=(a+b+c)/2.0;
    double ar=sqrt(p*(p-a)*(p-b)*(p-c));
    return(ar);
}