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