#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
int main() {
	fixed(cout);
	cout << setprecision(13);
	long a, l, x;
	while (cin >> a >> l >> x) {
		double h_1 = sqrt(l * l - a * a / 4.0);
		double h_2 = sqrt((x + l) * (x + l) / 4.0 - l * l / 4.0);
		cout << a * h_1 / 2 + l * h_2 << endl;
	}

	return 0;
}