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

double heron(double a, double b, double c) {
	const double s = (a + b + c) / 2.0;
	return sqrt(s * (s - a) * (s - b) * (s - c));
}

int main() {
	for(int a, l, x; scanf("%d %d %d", &a, &l, &x) == 3;) {
		const double half = (l + x) / 2.0;
		const double ans = heron(a, l, l) + 2.0 * heron(l, half, half);
		printf("%.10lf\n", ans);
	}

	return EXIT_SUCCESS;
}