#include <bits/stdc++.h>
#define eb emplace_back
#define mt make_tuple
#define mp make_pair
#define fi first
#define se second

using namespace std;
typedef long long ll;
typedef double db;
typedef pair<int, int> pi;
typedef tuple<int, int, int> tu;
bool Mbe;

const int N = 550;
int n, d, r;
db f[N][N], g[N][N], C[N][N];

void solve() {
    cin >> n >> d >> r;
    C[0][0] = 1;
    for (int i = 1; i <= n; i++) {
        C[i][0] = 1;
        for (int j = 1; j <= i; j++)
            C[i][j] = C[i - 1][j - 1] + C[i - 1][j];
    }
    f[n][0] = 1, g[n][0] = r;
    for (int i = n; i; i--) {
        for (int j = i; j; j--) {
            for (int s = 0; s + j <= d; s++) {
                f[j][s + j] += f[i][s] * C[i][j];
                g[j][s + j] += (g[i][s] + f[i][s] * min(j, r)) * C[i][j];
            }
        }
    }
    db a = 0, b = 0;
    for (int i = 1; i <= n; i++) a += f[i][d], b += g[i][d];
    cout << fixed << setprecision(6) << b / a << '\n';
}

bool Med;
int main() {
	ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
	cerr << (&Mbe - &Med) / 1048576.0 << " MB\n";
	#ifdef FILE
		freopen(".in", "r", stdin);
		freopen(".out", "w", stdout);
	#endif
	int T = 1;
	// cin >> T;
	while (T--) solve();
	cerr << (int)(1e3 * clock() / CLOCKS_PER_SEC) << " ms\n";
	return 0;
}