#include <bits/stdc++.h>
using namespace std;
const int maxn = 51;
const int tot = maxn * maxn;
long long input[maxn];
long long dp[maxn][tot][maxn];
long long dp2[maxn][tot][maxn];
long long comb[maxn + 1][maxn + 1];
int N, M;
void init() {
  cin >> N;
  for (int i = 0; i < N; i++) {
    cin >> input[i];
  }
  cin >> M;
  comb[0][0] = 1;
  for (int i = 1; i <= maxn; i++) {
    comb[i][0] = 1;
    for (int j = 1; j <= i; j++) {
      comb[i][j] = comb[i - 1][j] + comb[i - 1][j - 1];
    }
  }
}
long long factorial[maxn];
bool vst[maxn];
void run() {
  int total = 0;
  for (int i = 0; i < N; i++) {
    total += input[i];
  }
  if (total <= M) {
    cout << N << endl;
    return;
  }
  for (int i = 0; i < N; i++) {
    memset(vst, 0, sizeof(vst));
    vst[i] = 1;
    dp[i][0][0] = 1;
    int cnt = 0;
    for (int j = 0; j < N; j++) {
      if (vst[j] == 0) {
        cnt++;
        for (int k = 0; k < tot; k++) {
          for (int t = 0; t <= cnt; t++) {
            dp2[i][k][t] = dp[i][k][t];
          }
        }
        for (int k = 0; k < tot; k++) {
          for (int t = 0; t <= cnt; t++) {
            if (dp[i][k][t] && (k + input[j] < tot)) {
              dp2[i][k + input[j]][t + 1] += dp[i][k][t];
            }
          }
        }
        for (int k = 0; k < tot; k++) {
          for (int t = 0; t <= cnt; t++) {
            dp[i][k][t] = dp2[i][k][t];
          }
        }
      }
    }
  }
  double ans = 0;
  for (int i = M; i >= 0; i--) {
    for (int j = 0; j < N; j++) {
      if (i + input[j] > M) {
        for (int k = 0; k < N; k++) {
          double tmp = dp[j][i][k] * 1.0 / comb[N][k];
          tmp /= (N - k);
          tmp *= k;
          ans += tmp;
        }
      }
    }
  }
  printf("%.8lf\n", ans);
}
void baoli() {
  const int n = 50;
  int tmp[n];
  cout << n << " ";
  for (int i = 0; i < n; i++) {
    tmp[i] = i + 1;
    cout << tmp[i] << " ";
  }
  int m = 100;
  cout << m << endl;
  double ans = 0;
  for (int test = 0; test <= 10000; test++) {
    random_shuffle(tmp, tmp + n);
    int sum = 0;
    for (int i = 0; i < n; i++) {
      sum += tmp[i];
      if (sum > m) {
        ans += i;
        break;
      }
    }
  }
  cout << ans / 10000 << endl;
}
int main() {
  init();
  run();
  return 0;
}
