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

double r, h, d;

//断面積S
double S(double x){
  return 2*x*x*acos(d/(2.0*x))-d*sqrt(x*x-d*d/4);
}

int main(){
  cin >> r >> h >> d;
  double range = r - d*0.5;
  int num = 10000000;
  double dr = range / num;
  double vol = 0;
  for(int i = 0; i < num; i++){
    vol += h/r*(S(d*0.5+i*dr)+S(d*0.5+(i+1)*dr))*dr/2;
  }
  printf("%.10f\n",vol);
  return 0;
}