#include <iostream>
#include <string>
#include <algorithm>
#include <utility>
#include <cmath>
#include <cstdlib>
#include <map>
#include <set>
#include <queue>
#include <vector>
#include <iomanip>
#include <limits>
using namespace std;

int main() {
	long double a, b, c; cin >> a >> b >> c;
	long double s = (a + b + c) / 2;
	long double ans = s * (s - a) * (s - b) * (s - c);
	cout << fixed << setprecision(7);
	cout << sqrt(ans) / 4 << endl;
}