#define rep(i,n) for (int i=0;i < (int)(n);i++)
#define ALL(v) v.begin(),v.end()
typedef long long ll;

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

double r,h,d;
double f(double x) {
  return 2*r*r/(h*h)*x*x*acos(d*h/(2*r*x))-d/h*sqrt(4*r*r*x*x-d*d*h*h)/2;
}

int main() {
  cin>>r>>h>>d;
  
  double A=d*h/(2*r);
  double B=h;
  int N=100000;

  const double delta = (B - A) / N;
  double value = 0.0;
    
  for(int i=1; i<N; i++) {
    double x = A + i * delta;      
    value += ( f(x) + f(x+delta) + 4.0 * f(x+delta/2.0) ) * delta / 6.0;
  }
  cout<<fixed<<setprecision(15)<<value<<endl;
    
  return 0;
}