import java.util.*;
import java.io.*;
public class Main {
public static void main(String args[]){
	Scanner sc=new Scanner(System.in);
	double a=sc.next();
	double b=sc.next();
    double c=sc.next();
    double p=(a+b+c)/2.0;
    double ans =Math.sqrt(p*(p-a)*(p-b)*(p-c));
	System.out.println("" +ans);
}
}