#include <cmath>
#include <cstdio>
#include <iostream>
using namespace std;

int main() {
	double a, l, x;
	while (cin >> a >> l >> x) {
		double y = sqrt(l * l - (a / 2) * (a / 2));
		double h = (l + x) / 2.;
		double z = sqrt(h * h - (l / 2) * (l / 2));
		printf("%.10lf\n", a * y / 2. + l * z);
	}
	return 0;
}