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

int main() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    double r, h, d;
    cin >> r >> h >> d;
    double a = r - d / 2;
    double k = 1 - a / r;
    cout << setprecision(9) << h * r * r / 3 * (M_PI / 2 - 2 * k * sqrt(1 - k * k) - asin(k) + k * k * k * acosh(1 / k)) * 2 << '\n';
    return 0;
}