1131. The Circumference Of The Circle

import java.io.*;

public class Main {

    /**
     * http://acm.tju.edu.cn/toj/showp1131.html
     * @author hunglee
     * @throws IOException 
     */
    public static void main(String[] args) throws IOException {

        StreamTokenizer in = new StreamTokenizer(new BufferedReader(
                new InputStreamReader(System.in)));
        
        final double PI = 3.141592653589793;
        double x1, x2, x3, y1, y2, y3;
        double x, y, dx, dy, d, a1, b1, c1, a2, b2, c2, r, c;
        
        while (in.nextToken() != StreamTokenizer.TT_EOF) {
            x1 = in.nval;
            in.nextToken();    y1 = in.nval;
            in.nextToken();    x2 = in.nval;
            in.nextToken();    y2 = in.nval;
            in.nextToken();    x3 = in.nval;
            in.nextToken();    y3 = in.nval;
            
            a1 = 2.0 * (x2 - x1);
            b1 = 2.0 * (y2 - y1);
            c1 = x2 * x2 + y2 * y2 - x1 * x1 - y1 * y1;
            a2 = 2.0 * (x3 - x1);
            b2 = 2.0 * (y3 - y1);
            c2 = x3 * x3 + y3 * y3 - x1 * x1 - y1 * y1;
            
            dx = c1 * b2 - c2 * b1;
            dy = a1 * c2 - a2 * c1;
            d = a1 * b2 - a2 * b1;
            x = dx / d;
            y = dy / d;
            
            r = Math.sqrt((x - x1) * (x - x1) + (y - y1) * (y - y1));
            c = 2.0 * r * PI;
            System.out.println(String.format("%.2f", c));
        }
    }

}