#include <bits/stdc++.h>
using namespace std;
int64_t mod = 1e9 + 7, inf = 4e18;
map<vector<int64_t>, long double> seen;
long double rec(vector<int64_t> &people, int64_t n, int64_t len) {
  if (len < 0) return -1;
  if (n == 0) return 0;
  if (seen[people]) return seen[people];
  long double ans = 0;
  for (int64_t i = 0; i < people.size(); i++) {
    if (people[i] == 0) continue;
    people[i] -= 1;
    ans += (1 + rec(people, n - 1, len - i)) * (people[i] + 1) / n;
    people[i] += 1;
  }
  seen[people] = ans;
  return ans;
}
int32_t main() {
  ios::sync_with_stdio(false), cin.tie(nullptr);
  cout << fixed << setprecision(5);
  int64_t n, p;
  cin >> n;
  vector<int64_t> hist(51, 0);
  int64_t tmp;
  for (int64_t i = 0; i < int64_t(n); ++i) cin >> tmp, hist[tmp]++;
  cin >> p;
  cout << rec(hist, n, p);
  return 0;
}
