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

double area(double a, double b, double c) {
    double s = (a + b + c) / 2;
    return sqrt(s * (s - a) * (s - b) * (s - c));
}

int main() {
    int a, l, x;
    while (cin >> a >> l >> x) {
        double s = area(a, l, l) + 2 * area(l, (l + x) / 2.0, (l + x) / 2.0);
        printf("%.6f\n", s);
    }

}