#include <bits/stdc++.h>
using namespace std;
int main() {
  long double a, l, x;
  long double temp1, temp2, temp3;
  long double center, sides;
  while (cin >> a >> l >> x) {
    temp1 = sqrt(pow(l, 2) - pow(a / 2, 2));
    center = a * temp1 / 2;
    temp2 = sqrt(pow((l + x) / 2, 2) - pow(l / 2, 2));
    sides = temp2 * l;
    printf("%.10f\n", center + sides);
  }
}
