#include<iostream>
#include<iomanip>
#include<cmath>
using namespace std;
int a,b,c;
main()
{
	cin>>a>>b>>c;
	double s=(a+b+c)*.5;
	cout<<fixed<<setprecision(16)<<sqrt(s*(s-a)*(s-b)*(s-c))/4<<endl;
}