#include <bits/stdc++.h>
using namespace std;

int main() {
    double r, h, d;
    cin >> r >> h >> d;
    d /= 2 * r;
    const int split = 1000000;
    double s = 0;
    for (int i = 1; i <= split; i++) {
        double r_i = double(i) / split, v = i % 2 * 2 + 2;
        if (i == split) v = 1;
        if (d >= r_i) continue;
        double theta = acos(d / r_i);
        s += 2 * v * (r_i * r_i * theta - d * r_i * sin(theta));
    }
    cout << setprecision(17) << s * r * r * h / (3 * split) << endl;
}