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

double area(double a, double b)
{
	return (sqrt(a * a - b * b) * b / 2);
}

int main(void)
{
	int a, l, x;
	double s;
	
	while (scanf("%d%d%d", &a, &l, &x) != EOF){
		s = area(l, (double)a / 2) * 2;
		s += area((double)(l + x) / 2, (double)l / 2) * 4;
		printf("%.10lf\n", s);
	}
	
	return (0);
}