#include<iostream>
#include<cstdio>
#include<cmath>
#define loop(i,a,b) for(int i=a;i<b;i++)
#define rep(i,a) loop(i,0,a)
using namespace std;

int main(){
	double a,l,x;
	while(cin>>a>>l>>x){
		double ans=a*sqrt(4*l*l-a*a)/4+l*sqrt(x*(2*l+x))/2;
		printf("%.9f\n",ans);
	}
	return 0;
}