#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;

int main(){
	double a,l,x;
	while(cin >> a >> l >> x){
		double s=0;
		s += a*sqrt(l*l-a*a/4)/2;
		double L = (x+l) / 2.0;
		s +=  l * sqrt( L*L -l*l/4);
		printf("%.10lf\n",s);
	}
}