#include <iostream>
#include <string>
#include <algorithm>
#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <cmath>
#include <stdio.h>
#include <queue>
#include <deque>
#include <cstdio>
#include <set>
#include <map>
#include <bitset>
#include <stack>
#include <cctype>
using namespace std;
int main() {
	long double a, b, c;
	cin >> a >> b >> c;
	long double s = (a + b + c) / 2;
	long double m = sqrtl(s * (s - a) * (s - b) * (s - c));
	cout << fixed << setprecision(16) << m / 4 << endl;
}