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

using namespace std;

double helon(double a, double b, double c){
	double s = (a + b + c) / 2;
	return sqrt(s * (s - a) * (s - b) * (s - c));
}

double a, l, x;

int main(){
	while(~scanf("%lf%lf%lf", &a, &l, &x)){
		printf("%.10lf\n", helon(a, l, l) + helon(l, (l+x)/2, (l+x)/2) * 2);
	}
	return 0;
}