#include <boost/math/constants/constants.hpp>
#include <boost/math/quadrature/gauss.hpp>
#include <cmath>
#include <iostream>

int main() {
  double R, H, D;
  std::cin >> R >> H >> D;
  auto r = [=](const double& y) {
    return R * y / H;
  };
  auto t = [=](const double& y) {
    auto r_y = r(y);
    return boost::math::constants::pi<double>() - std::acos(1.0 - D * D / (2.0 * r_y * r_y));
  };
  auto s = [=](const double& y) {
    auto r_y = r(y);
    auto t_y = t(y);
    return r_y * r_y * (t_y - std::sin(t_y));
  };
  double a = H * D / (2.0 * R);
  double b = H;
  double ans = boost::math::quadrature::gauss<double, 10000>::integrate(s, a, b);
  std::cout << std::fixed << std::setprecision(10) << ans << std::endl;
}