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

int main(void){

	double a, l, x;
	while (cin >> a){
		cin >> l >> x;
		double s = (2 * l + a)/2;
		double A = sqrt(s*(s - a)*(s - l)*(s - l));
		s = (2 * l + x) / 2;
		double B = sqrt(s*(s - l)*(s - (l + x) / 2)*(s - (l + x) / 2));
		
		printf("%.6f\n", A + 2*B);
	}


	return 0;
}