#include <bits/stdc++.h>
#include <atcoder/all>

typedef unsigned long long ULLONG;
typedef long long LLONG;
static const LLONG MOD_NUM = 1000000007LL;//998244353LL;
static const LLONG INF_NUM = MOD_NUM * 20LL;
template<class _T> static void get(_T& a) {
	std::cin >> a;
}
template<class _T> static void get(_T& a, _T& b) {
	std::cin >> a >> b;
}
template<class _T> static void get(_T& a, _T& b, _T& c) {
	std::cin >> a >> b >> c;
}
template <class _T> static _T tp_abs(_T a) {
	if (a < (_T)0) {
		a *= (_T)-1;
	}
	return a;
}
template <class _T> static _T tp_pow(int x, int exp)
{
	_T ans = 1;
	_T base = x;
	while (exp > 0) {
		if (exp & 1) {
			ans = (ans * base);
		}
		base = (base * base);
		exp >>= 1;
	}
	return ans;
}

static void task();

int main()
{
	task();
	fflush(stdout);
	return 0;
}

static void task()
{
	double a, b, c;
	get(a, b, c);

	double s = (a + b + c) / 2.0;
	double S = sqrt(s * (s - a) * (s - b) * (s - c));

	printf("%.9lf\n", S / 4.0);
}