#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;

int main()
{
    double r, h, d;
    cin>>r>>h>>d;
    double ans=0;
    const int loop=10000000;
    double h1=h/loop;
    for(double i=h1/2; i<=h; i+=h1){
        double r1=r*i/h;
        if(d>=2*r1) continue;
        double t=acos(d/(2*r1))*2;
        ans+=h1*r1*r1*(t-sin(t));
    }
    printf("%.7lf\n", ans);
    return 0;
}