#include<iostream>
#include<iomanip>
#include<cmath>
using namespace std;

int main() {
	double a,b,c;
	cin>>a>>b>>c;
	if(a+b > c && a+c > b && b+c>a) {
		double p = (a+b+c) / 2;
		double s = sqrt(p*(p-a)*(p-b)*(p-c));
		cout<<fixed<<setprecision(1)<<s;
	} else cout<<0;
	return 0;
}