#include <stdio.h>
#include <stdlib.h>
main(){
double *x1_ptr, *x2_ptr, *x3_ptr, *y1_ptr, *y2_ptr, *y3_ptr;
double x1val, x2val, x3val, y1val, y2val, y3val, area;
x1_ptr = &x1val;
x2_ptr = &x2val;
x3_ptr = &x3val;
y1_ptr = &y1val;
y2_ptr = &y2val;
y3_ptr = &y3val;
printf("x1="); scanf("%le", x1_ptr);
printf("y1="); scanf("%le", y1_ptr);
printf("x2="); scanf("%le", x2_ptr);
printf("y2="); scanf("%le", y2_ptr);
printf("x3="); scanf("%le", x3_ptr);
printf("y3="); scanf("%le", y3_ptr);
area = ((*x3_ptr)-(*x2_ptr) * (*y1_ptr)-(*y2_ptr) -
(*x1_ptr)-(*x2_ptr) * (*y3_ptr)-(*y2_ptr)) * 0.5;
printf("area = %lf\n", area);
}