#include <iostream>
#include <cmath>
#include <math.h>
#include <iomanip>
using namespace std;
int main(){
	int a,b,c,p;
	cin>>a>>b>>c;
	p=(a+b+c)/2.0;
	double s;
	s=sqrt(p*(p-a)*(p-b)*(p-c));
	cout<<fixed<<setprecision(1)<<s<<endl;
	return 0;
}