#include<stdio.h>
#include<math.h>
int main(void)
{
	int a,l,x;
	double s,s2,x2;
	
	while(1)	{
		if(scanf("%d %d %d",&a,&l,&x)==EOF)	break;
		s=(double)(a+l+l)/2.0;
		s2=sqrt(s*(s-l)*(s-l)*(s-a));
//		printf("s=%lf\n",s2);
		s=(double)(l+l+x)/2.0;
		x2=(x+l)/2.0;
		s2+=2.0*sqrt(s*(s-l)*(s-x2)*(s-x2));
		printf("%lf\n",s2);
	}
	return 0;
}
