#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
using vi = vector<ld>;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int n;
  cin >> n;
  vector<int> a(n);
  for (int& e : a) cin >> e;
  int p;
  cin >> p;
  vector<vi> dp(n + 1, vi(p + 1));
  dp[0][0] = 1;
  for (int& cur : a) {
    for (int j = n - 1; j >= 0; j--) {
      for (int s = 0; s + cur <= p; s++) {
        dp[j + 1][s + cur] += (dp[j][s] / (n - j)) * (j + 1);
      }
    }
  }
  ld ans = 0;
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j <= p; j++) {
      ans += dp[i][j];
    }
  }
  cout << fixed << setprecision(10) << ans << endl;
}
