#include<bits/stdc++.h>
using namespace std;

int main()
{
	int a ,b , c ,p;
	cin >> a >>  b >> c;
	p = (a + b + c) / 2.0;
	double sum = sqrt(p * (p - a) * (p - b) * (p - c));
	cout << setprecision(1) << fixed << sum << endl;
}