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;
}

Java :

import java.text.DecimalFormat;
import java.util.Scanner;

public class Main {
   private static Scanner s = new Scanner(System.in) ;
   private static double PI = 3.141592653589793 ;
   private static DecimalFormat df = new DecimalFormat("0.00") ;
   public static void main(String[] args) {
	  while(s.hasNext()){
		  double x1 = s.nextDouble() ;
		  double y1 = s.nextDouble() ;
		  double x2 = s.nextDouble() ;
		  double y2 = s.nextDouble() ;
		  double x3 = s.nextDouble() ;
		  double y3 = s.nextDouble() ;
		  
		  double d = 2*(x1-x2)*(y3-y2) - 2*(y1-y2)*(x3-x2) ;
		  double d1 =  (y3-y2)*(x1*x1+y1*y1-x2*x2-y2*y2) - (y1-y2)*(x3*x3+y3*y3-x2*x2-y2*y2);
		  double d2 = (x1-x2)*(x3*x3+y3*y3-x2*x2-y2*y2) - (x3-x2)*(x1*x1+y1*y1-x2*x2-y2*y2);
		  
		  double x = d1/d ;
		  double y = d2/d ;
		  double R = Math.sqrt((x2-x)*(x2-x)+(y2-y)*(y2-y)) ;
		  
		  System.out.println(df.format(PI*R*2));
	  }
   }
}