#include <iostream>
#include <complex>
#include <cmath>
#include <cstdio>
#include <string>
#include <sstream>
#include <iomanip>

using namespace std;

double heron(double a, double b, double c) {
    double s = (a + b + c) / 2.0;
    return sqrt(s * (s - a) * (s - b) * (s - c));
}

int main() {
    double a, l, x;
    cout << setprecision(20);
    while(cin >> a >> l >> x) {
        cout << heron(l, l, a) + 2 * heron(l, (l + x) / 2.0, (l + x) / 2.0) << endl;
    }
    return 0;
}