#include <bits/stdc++.h>
using namespace std;
int n, p, s;
double res;
int a[57];
double fact[57];
long long dp[57][57];
long long pref[57];
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  fact[0] = log(1);
  cin >> n;
  for (int i = 1; i <= n; ++i) {
    cin >> a[i];
    fact[i] = fact[i - 1] + log(i);
    s += a[i];
  }
  cin >> p;
  if (s <= p) {
    cout << fixed << setprecision(10) << n;
    return 0;
  }
  for (int b = 1; b <= n; ++b) {
    for (int i = 0; i <= n; ++i) {
      for (int j = 0; j <= p; ++j) {
        dp[i][j] = 0;
      }
    }
    dp[0][0] = 1;
    for (int i = 1; i <= n; ++i) {
      if (i == b) continue;
      for (int j = n - 1; j >= 0; --j) {
        for (int k = 0; k + a[i] <= p; ++k) {
          dp[j + 1][k + a[i]] += dp[j][k];
        }
      }
    }
    for (int j = 0; j <= n; ++j) {
      for (int k = p - a[b] + 1; k <= p; ++k) {
        pref[j] += dp[j][k];
      }
    }
  }
  for (int i = 0; i < n; ++i) {
    res += exp(log(i) + log(pref[i]) + fact[i] + fact[n - i - 1] - fact[n]);
  }
  cout << res;
}
