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

int main(void) {
	int a,l,x;
	double center_h,center_s;
	double mawari_h,mawari_s;
	while(scanf("%d%d%d",&a,&l,&x)==3) {
		center_h=sqrt(l*l-(a/2.0)*(a/2.0));
		center_s=a*center_h/2.0;
		mawari_h=sqrt(((l+x)/2.0)*((l+x)/2.0)-(l/2.0)*(l/2.0));
		mawari_s=l*mawari_h/2.0;
		printf("%.15f\n",center_s+mawari_s*2.0);
	}
	return 0;
}