#include <bits/stdc++.h>
using namespace std;
double a, l, x;
int main() {
  while (cin >> a >> l >> x) {
    double h = sqrt(x * x / 4 + x * l / 2);
    double area = l * h + 0.5 * a * sqrt(l * l - a * a / 4);
    printf("%.8f\n", area);
  }
}
