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

using namespace std;

double S(double a, double b, double c) {
	double z = (a + b + c) / 2;
	return sqrt(z * (z-a) * (z-b) * (z-c));
}

int main() {
	int a, l, x;
	while (scanf("%d%d%d", &a, &l, &x) != EOF) {
		double b = (l+x) / 2.0;
		printf("%.8f\n", S(a, l, l) + 2 * S(l, b, b));
	}
}