#include<iostream>
#include<cmath>
#include<algorithm>
#include<cstdio>
using namespace std;
int main(){

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

	}
	return 0;
}