#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep2(i, x, n) for(int i = x; i <= n; i++)
#define rep3(i, x, n) for(int i = x; i >= n; i--)
#define each(e, v) for(auto &e: v)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) (int)x.size()
using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
const int MOD = 1000000007;
//const int MOD = 998244353;
const int inf = (1<<30)-1;
const ll INF = (1LL<<60)-1;
template<typename T> bool chmax(T &x, const T &y) {return (x < y)? (x = y, true) : false;};
template<typename T> bool chmin(T &x, const T &y) {return (x > y)? (x = y, true) : false;};

struct io_setup{
    io_setup(){
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout << fixed << setprecision(15);
    }
} io_setup;

const double EPS = 1e-10;

int main(){
    double R, H, D;
    cin >> R >> H >> D;
    double A = H*D/(2*R)+EPS, B = H;
    if(A > B) {cout << 0.0 << '\n';; return 0;}

    int N = 1000000;
    double ans = 0, dx = (B-A)/N;
    rep2(i, 0, N){
        double x = A+(B-A)*i/N;
        double r = (R*x)/H;
        if(2*r-D < EPS) continue;
        double the = acos(D/(2*r));
        double S = r*r*(2*the-sin(2*the));
        if(i == 0 || i == N) S *= 0.5;
        ans += S*dx;
    }

    cout << ans << '\n';
}