#include <cmath>
#include <iomanip>
#include <iostream>
using namespace std;
int main() {
    double R, H, D; cin >> R >> H >> D;
    double ans = 0;
    const double hmax = H * (1 - D / 2 / R),
        dh = H * 1e-7;
    for (double h = dh * 0.5; h < hmax; h += dh) {
        const double rh = R * (1 - h / H),
            theta = 2 * acos(D / 2 / rh);
        ans += dh * rh * rh * (theta - sin(theta));
    }
    cout << setprecision(12) << ans << endl;
}