#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  int a, b, c;
  cin >> a >> b >> c;
  double s = (a + b + c) / 2.0;
  double area = sqrt(s * (s - a) * (s - b) * (s - c));
  cout << fixed << setprecision(15) << area / 4 << '\n';
  return 0;
}