#include<iostream>
#include<cmath>
#include<cstdio>

using namespace std;


int main(){
	double a, l, x;
	double temp1, temp2;
	double center, sides;


	while (cin >> a >> l >> x){
		temp1 = sqrt(pow(l, 2) - pow(a / 2, 2));
		center = a * temp1 / 2;
		
		temp2 = sqrt(pow((l + x) / 2, 2) - pow(l / 2,2));
		sides = temp2 * l;

		printf("%10.10f\n", center + sides);
	}
}