#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")

#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = (n)-1; i >= 0; --i)
#define chmax(a, b) a = max(a, b)
#define chmin(a, b) a = min(a, b)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(12);
    double rmax, h, d;
    cin >> rmax >> h >> d;
    const double rmin = d / 2;
    constexpr int num = 5e7;
    const double rstep = (rmax - rmin) / num;
    double ans = 0;
    rep(i, num) {
    //for(double r = rmin; r < rmax; r += rstep) {
        const double r = rmin + rstep * i;
        double t = acos(d / 2 / r);
        //ans += r * r * (2 * t - sin(2 * t));
        double x = 2 * t;
        double x2 = x * x;
        ans += r * r * (x2 * x / 6 * (1 - x2 / 20 * (1 - x2 / 42 * (1 - x2 / 72 * (1 - x2 / 110)))));
    }
    ans *= rstep * h / rmax;
    cout << ans << endl;
}

/*
int main() {
    constexpr int attempts = 1e8;
    int hits = 0;
    double R, H, D;
    cin >> R >> H >> D;
    std::default_random_engine generator(std::chrono::system_clock::now().time_since_epoch().count());
    //std::default_random_engine generator(0);
    // [-R, D + R] * [-R, R] * [0, H]
    std::uniform_real_distribution<> xdist(-R, D + R), ydist(-R, R), zdist(0, H);
    const double RH = R / H;
    for(int i = attempts; i; i--) {
        const double x = xdist(generator), y = ydist(generator), z = zdist(generator);
        const double dl = x * x + y * y, dr = (x - D) * (x - D) + y * y, r = z * RH;
        const double r2 = r * r;
        if (dl <= r2 && dr <= r2) hits++;
    }
    cout << fixed << setprecision(12);
    cout << double(D + 2 * R) * 2 * R * H * hits / attempts << endl;
}
*/