1. 
package acm_438_the_circumference_of_the_circle;

/**
 * FWP, AusgewŠhlte Probleme aus dem ACM Programming Contest, SS10 
 * Problem: 438 - The circumference of the circle
 * Link: http://uva.onlinejudge.org/index.php?option=onlinejudge&page=show_problem&problem=379
 * 
 * @author Martin Lambeck
 * @version 1.0, 01/08/2010
 * 
 * Method : geometry
 * Status : Accepted
 * Runtime: 0.240
 */

import java.io.BufferedInputStream;
import java.util.Scanner;

public class Main
{
	final static double PI = 3.141592653589793d;
	static Point p1;
	static Point p2;
	static Point p3;

	static Scanner sc = new Scanner(new BufferedInputStream(System.in));

	public static void main (String...args)
	{
		while (testcase());
	}

	public static boolean testcase()
	{
		double x, y;
		double frac, denom;
		double r;
		
		
		if (!sc.hasNextDouble())
			return false;

		p1 = new Point(sc.nextDouble(), sc.nextDouble());
		p2 = new Point(sc.nextDouble(), sc.nextDouble());
		p3 = new Point(sc.nextDouble(), sc.nextDouble());
		
		Point m1 = mid(p1, p2); //middle point of p1->p2
		Point m2 = mid(p2, p3); //middle point of p2->p3
		
		Vector v1 = ortho(m1, p2); //v1 orthogonal to m1->p2 
		Vector v2 = ortho(m2, p3); //v2 orthogonal to m2->p3
		
		
		// m1 + t * v1 = m2 + s * v2
		//
		// I)   m1.x + t * v1.x = m2.x + s * v2.x
		// II)  m1.y + t * v1.y = m2.y + s * v2.y
		//
		// t = ... * s
		// eliminate t => s = ... = frac
		//
		// x,y = center of circle = m2 + v2 * s = m2 + v2 * frac
		
		denom = (v1.y * v2.x - v1.x * v2.y);
		frac = (v1.x * (m2.y - m1.y) - v1.y * (m2.x - m1.x)) / denom;
		
		if (denom == 0)
			throw new IllegalArgumentException(); //wont happen, there is no such testcase
		
		x = m2.x + v2.x * frac;
		y = m2.y + v2.y * frac;
		
		
		// circumference = r * 2 pi = | p1->(center) | * 2 pi = sqrt( (p1.x - x)^2 + (p1.y - y)^2 ) * 2 pi
		
		r = Math.sqrt(Math.pow(p1.x - x, 2) + Math.pow(p1.y - y, 2));
		
		System.out.printf("%.2f%n", r * 2 * PI);
		
		return true;
	}
	
	public static Point mid(Point p1, Point p2)
	{
		double x = (p2.x - p1.x) / 2.0 + p1.x;
		double y = (p2.y - p1.y) / 2.0 + p1.y;
		
		return new Point (x, y);
	}
	public static Vector ortho(Point p0, Point q)
	{
		double x = q.x - p0.x;
		double y = q.y - p0.y;

		return new Vector(y, -x);
	}

}

class Point
{
	double x;
	double y;
	
	public Point(double xx, double yy)
	{
		x = xx;
		y = yy;
	}
}

class Vector
{
	double x;
	double y;
	
	public Vector(double xx, double yy)
	{
		x = xx;
		y = yy;
	}
}

2.

/**
 * FWP, AusgewŠhlte Probleme aus dem ACM Programming Contest, SS10 
 * Problem: 438 - The circumference of the circle
 * Link: http://uva.onlinejudge.org/index.php?option=onlinejudge&page=show_problem&problem=379
 * 
 * @author Michael Haus
 * @version 1.0, 02/08/2010
 * 
 * Method : geometry
 * Status : Accepted
 * Runtime: 0.240
 */

import java.util.Scanner;

public class Main {

	public static void main(String[] args){

		Scanner input = new Scanner(System.in);
		while(input.hasNext()){

			double ax = input.nextDouble(); 
			double ay = input.nextDouble();
			
			double bx = input.nextDouble();
			double by = input.nextDouble();
			
			double cx = input.nextDouble();
			double cy = input.nextDouble();
			
			double[] eins = new double[]{1,-1*ax,-1*ay,-1*(ax*ax+ay*ay)};
			double[] zwei = new double[]{1,-1*bx,-1*by,-1*(bx*bx+by*by)};
			double[] drei = new double[]{1,-1*cx,-1*cy,-1*(cx*cx+cy*cy)};
			
			for(int i = 0; i < eins.length; i++){
				zwei[i] = zwei[i] - eins[i];
				drei[i] = drei[i] - eins[i];
			}
			
			//Cramersche Regel
			double detA = (zwei[1]*drei[2])-(zwei[2]*drei[1]);
			double detA1 = (zwei[3]*drei[2])-(zwei[2]*drei[3]);
			double detA2 = (zwei[1]*drei[3])-(zwei[3]*drei[1]);
			
			//b ,c
			double b = detA1/detA;
			double c = detA2/detA;
			
			//a
			eins[1] = eins[1] * b;
			eins[2] = eins[2] * c;
			
			double a = (eins[3]-eins[1])-eins[2];
			
			double x = b/2;
			double y = c/2;
			
			double r = Math.sqrt(x*x+y*y-a);
			
			System.out.println(Math.round(2*Math.PI*r * 100.)/100.);
		}
	}	
}