#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9;
const long long MOD = 1e9 + 7;
double EPS = 1e-8;
const double PI = acos(-1);
double helon(double a, double b, double c) {
  double ret = 0.0;
  double s = (a + b + c) / 2;
  ret = sqrt(s * abs(a - s) * abs(b - s) * abs(c - s));
  return ret;
}
int main() {
  int a, l, x;
  while (cin >> a >> l >> x) {
    double s = helon(a, l, l) + 2 * max({helon(l, (l + x) / 2.0, (l + x) / 2.0),
                                         helon(x, l, l)});
    printf("%.15lf\n", s);
  }
}
