#include<iostream>
#include<cstdio>
#include<cmath>
using namespace std;


int main(){
	int a,l,x;
	while(cin>>a>>l>>x&&a){
		double h=sqrt((l+x)*(l+x)-l*l)/2;
		double g=sqrt(l*l-(double)a*a/4);
		printf("%.8f\n",a*g/2+h*l);
	}
	return 0;
}