// Aizu Vol-10 1091: KND is So Sexy
// 2017.8.18 bal4u@uu

#include <stdio.h>
#include <math.h>

int main()
{
	double a, l, x, s, ans;

	while (scanf("%lf%lf%lf", &a, &l, &x) == 3) {
		s = l + a / 2.0;
		ans = a/2.0 * sqrt(s*(s - a));
		s = l + x / 2.0;
		ans += l * sqrt(s*x/2.0);
		printf("%.8lf\n", ans);
	}
	return 0;
}