#include<cmath>
#include<iostream>
#include<cstdio>
using namespace std;
int main(){
	double a,l,x;
	while(cin>>a>>l>>x){
		printf("%.10lf\n",sqrt(l*l-(a/2)*(a/2))*a/2+sqrt(((l+x)/2)*((l+x)/2)-(l/2)*(l/2))*l);
	}
	return 0;
}