#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define rep(i, n) for (int i = 0; i < (n); i++)
#define P pair<int, int>

int main() {
  double a, b, c;
  cin >> a >> b >> c;

  double s = (a+b+c)/2;
  double ans = sqrt(s*(s-a)*(s-b)*(s-c));
  ans /= 4;
  cout << fixed << setprecision(15) << ans << endl;



}