#include <bits/stdc++.h>
using namespace std;
const int N = 505;
int n, d, r;
double ans, tot;
double C[N][N], f[N][N], g[N][N];
int main() {
    scanf("%d%d%d", &n, &d, &r);
    f[0][n] = r;
    g[0][n] = 1;

    for (int i = 0; i <= n; ++i)
        for (int j = C[i][0] = 1; j <= i; ++j)
            C[i][j] = C[i - 1][j - 1] + C[i - 1][j];

    for (int j = 0; j <= d; ++j) {
        for (int k = 1; k <= n; ++k)
            if (g[j][k]) {
                for (int p = 1; p <= k && j + p <= d; ++p) {
                    g[j + p][p] += g[j][k] * C[k][p];
                    f[j + p][p] += f[j][k] * C[k][p] + g[j][k] * C[k][p] * min(p, r);
                }
            }
    }

    for (int p = 1; p <= n; ++p)
        ans += f[d][p], tot += g[d][p];

    printf("%.8lf\n", ans / tot);
    return 0;
}