#include <stdio.h>
#include <math.h>
#define pi 3.141592653589793
int main()
{
int i,j;
double x1,x2,x3,y1,y2,y3,r,k,b,x,y;
while(~scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3)){
if(y1!=y3&&y2!=y3){
k=(x3-x1)/(y1-y3)-(x3-x2)/(y2-y3);
b=(x3*x3-x1*x1)/2/(y1-y3)-(x3*x3-x2*x2)/2/(y2-y3)+(y2-y1)/2;
x=b/k;
y=(y1+y3)/2-(x3*x3-x1*x1)/2/(y1-y3)+x*(x3-x1)/(y1-y3);
// printf("3333333\n");
}else if(y1==y3&&y2!=y3)
{
x=(x1+x3)/2;
y=x/(y1-y2)*(x2-x1)+(y1+y2)/2-(x2*x2-x1*x1)/2/(y1-y2);
//printf("222222\n");
}else if(y2==y3&&y1!=y3)
{
x=(x2+x3)/2;
y=x/(y1-y2)*(x2-x1)+(y1+y2)/2-(x2*x2-x1*x1)/2/(y1-y2);
//printf("11111\n");
//printf("%.4lf,,%.4lf",x,y);
}
r=sqrt(pow(x-x1,2)+pow(y-y1,2));
printf("%.2lf\n",pi*2*r);
}
return 0;
}
外心是垂直平分线的交点，求垂直平分线时要注意垂直平分线平行于y轴的情况，此时这条直线没有斜率，所以开始前需要分情况讨论。求出外心坐标，求外心与任意一点的距离r，再求出2πr
