#include <iostream>
#include <cstdlib>
#include <cmath>
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);
  }
}