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

using namespace std;

int main() {
	double a, l, x;
	while (cin >> a >> l >> x) {
		x = (l+x) / 2.0;
		double s1, s2;
		double M1, M2;
		s1 = (a + l*2.0) / 2.0;
		s2 = (l + x*2.0) / 2.0;
		M1 = sqrt(s1 * (s1 - a) * (s1 - l) * (s1 - l));
		M2 = sqrt(s2 * (s2 - l) * (s2 - x) * (s2 - x));
		printf("%.10f\n", M1+M2*2);
	}
	return 0;
}