#include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;
long double a, l, x, p;
int main() {
	while (cin >> a >> l >> x) {
		long double z = sqrtl(l*l - a*a / 4); p = 0;
		p += z*a / 2;
		z = sqrtl((l + x)*(l + x) / 4 - l*l / 4);
		p += z*l;
		printf("%.15Lf\n", p);
	}
	return 0;
}