#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 = 3e7;
    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;
        double z = 1;
        for(int i = 20; i >= 4; i -= 2) {
            z = 1 - x2 / (i * (i + 1)) * z;
        }
        z = r * r * x * x2 / 6 * z;
        //ans += r * r * (x2 * x / 6 * (1 - x2 / 20 * (1 - x2 / 42 * (1 - x2 / 72 * (1 - x2 / 110 * (1 - x2 / 156))))));
        ans += z;
    }
    ans *= rstep * h / rmax;
    cout << ans << endl;
}