#include <iostream>
#include <cmath>
#include <cstdio>

using namespace std;

int main(int argc, char *argv[]) {
  double a, l, x;
  while (cin >> a >> l >> x) {
    double abc = a * l * sqrt(1 - a/2/l * a/2/l) / 2;
    double acd = l * (l+x)/2 * sqrt(1 - l/(l+x) * l/(l+x)) / 2;
    printf("%.10lf\n", abc + acd*2);
  }
  return 0;
}