#include "bits/stdc++.h"
using namespace std;
using ld = long double;
const ld eps = 1e-9;


int main() {
	cout << fixed << setprecision(10);
	ld  a, l, x;
	while (cin >> a >> l >> x) {
		ld h = sqrt(powl(l, 2) - powl(a / 2, 2));
		ld w = sqrt(powl((l + x) / 2, 2) - powl(l / 2, 2));
		ld ans = a*h / 2 + l*w;
		cout << ans << endl;
	}
	return 0;
}