zoj1090 The Circumference of the Circle

1. //http://acm.zju.edu.cn/show_problem.php?pid=1090
2. //zoj1090 The Circumference of the Circle
3. //Accepted 1090 C++ 00:00.00 396K
4. #include <stdio.h>
5. #include <math.h>
6. #define PI acos(-1.0)
7. int main()
8. {
9. #ifndef ONLINE_JUGDE
10.        freopen("1090.txt","r",stdin);
11. #endif
12.        double x1,y1,x2,y2,x3,y3,r,a,b,c,p,S,ans;
13.        while (scanf ("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3) != EOF){
14.               a = sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
15.               b = sqrt((x2-x3)*(x2-x3)+(y2-y3)*(y2-y3));
16.               c = sqrt((x3-x1)*(x3-x1)+(y3-y1)*(y3-y1));
17.               p = (a+b+c)/2;
18.               S = sqrt(p*(p-a)*(p-b)*(p-c));
19.               r = 0.25*a*b*c/S;
20.               ans = 2*PI*r;
21.               printf ("%.2lf/n",ans);
22.        }
23. #ifndef ONLINE_JUGDE
24.        fclose(stdin);
25. #endif
26.        return 0;
27. }