#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#include <time.h>
#include <stack>
#include <array>
#include <list>
#define popcount __builtin_popcount
using namespace std;
typedef long long ll;
typedef pair<int, int> P;
const double PI=acos(-1.0);
int main()
{
    double r, h, d;
    cin>>r>>h>>d;
    double t=2*acos(d/(2*r));
    double k=d/(2*r);
    double ans=2*h*r*r/3*(PI/2-2*k*sqrt(1-k*k)-asin(k)+k*k*k*acosh(1/k));
    printf("%.7lf\n", ans);
    return 0;
}