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

int main() {
    double a, l, x;
    while (cin >> a >> l >> x) {
        double dM = l * sqrt(x*x+2*l*x) * 0.5;
        double A = a * sqrt(l*l - a*a*0.25) * 0.5;
        printf("%.8lf\n", dM + A);
    }
    return 0;
}