#include<iostream>
#include<math.h>
#include<iomanip>
using namespace std;
int main(){
	double a,l,x;
	while(cin>>a>>l>>x){
		double b,c;
		b=(double)sqrt((double)l*l-a*a/4);
		c=(double)sqrt((double)(l+x)*(l+x)-l*l);
		cout<<fixed<<setprecision(10)<<a*b/2+l*c/2<<endl;
	}
	return 0;
}