// Arup Guha
// 2/11/2017
// Solution to 2016 German Programming Contest Problem G: Formula

import java.util.*;

public class g {
	
	public static void main(String[] args) {
		
		// Read in points.
		Scanner stdin = new Scanner(System.in);
		int[][] pts = new int[3][2];
		for (int i=0; i<3; i++)
			for (int j=0; j<2; j++)
				pts[i][j] = stdin.nextInt();
				
		// Read in radius.
		double r = stdin.nextDouble();
		
		// Use the cross product magnitude to get the real area of the triangle.
		int[] v1 = vect(pts[0], pts[1]);
		int[] v2 = vect(pts[0], pts[2]);
		int[] v3 = vect(pts[1], pts[2]);
		int twicearea = (int)Math.abs(v1[0]*v2[1]-v2[0]*v1[1]);
		
		// Then obtain the real radius.
		double realr = twicearea/(mag(v1)+mag(v2)+mag(v3));
		
		// Finally output the percentage error.
		System.out.println((realr-r)/r*100);
		
	}
	
	public static double mag(int[] a) {
		return Math.sqrt(a[0]*a[0]+a[1]*a[1]);
	}
	
	public static int[] vect(int[] a, int[] b) {
		return new int[]{b[0]-a[0], b[1]-a[1]};
	}
}