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

signed main() {
	double a, b, c;
	cin >> a >> b >> c;
	double s = (a + b + c) / 2;
	double area = sqrt(s * (s - a) * (s - b) * (s - c));
	printf("%.14f\n", area / 4);
	return 0;
}