import java.util.*;
import java.lang.*;
import java.io.*;
 
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		double a,b,c,d,f,p1,p2,S1,S2,S;
    	 	Scanner in = new Scanner(System.in);
		a = in.nextDouble();
		b = in.nextDouble();
		c = in.nextDouble();
		d = in.nextDouble();
		f = in.nextDouble();
		p1=(a+b+f)/2;
		p2=(c+d+f)/2;
		S = Math.sqrt(p1*(p1-a)*(p1-b)*(p1-f)) +  Math.sqrt(p2*(p2-c)*(p2-d)*(p2-f));
		System.out.printf("%.4f",S);
 
	}
}

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