#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <cassert>
#include <cmath>

#define debug_value(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << #x << "=" << x << endl;
#define debug(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << x << endl;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

using namespace std;
typedef long long ll;

double pow3(double x){ return x*x*x;}
double pow2(double x){ return x*x;}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    double r, h, d; cin >> r >> h >> d;
    double a = r-d/2.0;
    double k = 1.0-a/r;
    double v = (h*pow2(r)/3.0)*(
        M_PI/2.0 - 2*k*sqrt(1-pow2(k)) - asin(k) + pow3(k)*acosh(1.0/k)
    );
    cout << 2*v << endl;
}