ＮＹＯＪ 三角形面积

 
#include<stdlib.h>
#include<stdio.h>
#include<math.h>

int main()
{
    double a1,a2,b1,b2,c1,c2;
    while(scanf("%lf %lf %lf %lf %lf %lf",&a1,&a2,&b1,&b2,&c1,&c2),(a1||a2||b1||b2||c1||c2))
    {
      double a,b,c,s,q;
      
      a=sqrt((b1-c1)*(b1-c1)+(b2-c2)*(b2-c2));
      b=sqrt((a1-c1)*(a1-c1)+(a2-c2)*(a2-c2));
      c=sqrt((a1-b1)*(a1-b1)+(a2-b2)*(a2-b2));
      //s=sqrt((a+b+c)*(a+b-c)*(a-b+c)*(b+c-a));
      q=(a+b+c)/2;
      s=sqrt(q*(q-a)*(q-b)*(q-c));
      printf("%.1lf\n",s);
    }
}