C :

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

#define pi 3.141592653589793

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

int main()
{
	double x1,y1,x2,y2,x3,y3,a,b,c,r;
	while(scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3)!=EOF)
	{
		a=sqr(x1-x2)+sqr(y1-y2);
		b=sqr(x1-x3)+sqr(y1-y3);
		c=sqr(x2-x3)+sqr(y2-y3);
		r=sqrt(c/4/(1-sqr(a+b-c)/4/a/b));
		printf("%.2lf\n",2*pi*r);
	}
	return 0;
}

C++ :

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

#define pi 3.141592653589793

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

int main()
{
	double x1,y1,x2,y2,x3,y3,a,b,c,r;
	while(scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3)!=EOF)
	{
		a=sqr(x1-x2)+sqr(y1-y2);
		b=sqr(x1-x3)+sqr(y1-y3);
		c=sqr(x2-x3)+sqr(y2-y3);
		r=sqrt(c/4/(1-sqr(a+b-c)/4/a/b));
		printf("%.2lf\n",2*pi*r);
	}
	return 0;
}