/* -*- coding: utf-8 -*-
 *
 * 1319.cc:  No.1319 最強とんがりコーン - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int N = 5000000;
const double PI = acos(-1.0);

/* typedef */

/* global variables */

double ars[N + 1];

/* subroutines */

/* main */

int main() {
  double r, h, d;
  scanf("%lf%lf%lf", &r, &h, &d);

  double r0 = d * 0.5;
  double h0 = h * (1.0 - r0 / r), dh = h0 / N;

  for (int i = 0; i < N; i++) {
    double rz = r * (h - dh * i) / h;
    double th = acos(r0 / rz);
    ars[i] = rz * rz * th - r0 * r0 * tan(th);
    //printf("rz=%lf, th=%lf, ars[%d]=%lf\n", rz, th, i, ars[i]);
  }
  ars[N] = 0.0;
  
  double v = 0.0;
  for (int i = 0; i < N; i++)
    v += (ars[i] + ars[i + 1]) / 2 * dh;

  printf("%.15lf\n", v * 2);
  return 0;
}