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

using namespace std;

int main()
{
    string s;
    while (getline(cin, s)) {
        stringstream ss;
        ss << s;
        int a, l, x;
        ss >> a >> l >> x;
        auto r = l / 2.0 * sqrt(x * x + 2.0 * l * x) +
                 a / 4.0 * sqrt(4.0 * l * l - a * a);
        cout << setprecision(17) << r << endl;
    }
}