#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
int main() {
	double a, l, x;
	while (cin >> a >> l >> x) {
		double h_1 = sqrt(l * l - (a/2) * (a/2));
		double m = a * h_1 / 2;
		double h_2 = sqrt(((x + l)/2)* ((x + l)/2) - (l/2) * (l/2));
		double s = l * h_2 / 2;
		printf("%.6f\n", m + s * 2);
	}

	return 0;
}