#include <bits/stdc++.h>

#define all(v) (v).begin(), (v).end()
#define rep(i, n) for (ll i = 0; i < (ll)(n); ++i)
#define rep2(i, n, m) for (ll i = n; i <= (ll)(m); ++i)
#define rep3(i, n, m) for (ll i = n; i >= (ll)(m); --i)

using namespace std;

using ld = long double;
using ll = long long int;
using pl = pair<ll, ll>;
using vl = vector<ll>;
using vvl = vector<vl>;
using vpl = vector<pl>;

template <class T> using pqg = priority_queue<T, vector<T>, greater<T>>;
template <class T> using pq = priority_queue<T>;
template <class T> T chmax(T &a, const T &b) { return a = max(a, b); }
template <class T> T chmin(T &a, const T &b) { return a = min(a, b); }

const ll INF = 100000000000000000;
const ll MAX = 500005;
const ll MOD = 1000000007;
// const ll MOD = 998244353;

int main(void) {

  ld a, b, c;
  cin >> a >> b >> c;

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

  cout << fixed << setprecision(15) << S / 4.0 << endl;

  return (0);
}