#include <bits/stdc++.h>
using namespace std;
const int dr[] = {1, 0, -1, 0, 1, 1, -1, -1};
const int dc[] = {0, 1, 0, -1, 1, -1, -1, 1};
const double eps = 1e-9;
const int INF = 0x7FFFFFFF;
const long long INFLL = 0x7FFFFFFFFFFFFFFFLL;
const double pi = acos(-1);
template <class T>
T take(queue<T> &O) {
  T tmp = O.front();
  O.pop();
  return tmp;
}
template <class T>
T take(stack<T> &O) {
  T tmp = O.top();
  O.pop();
  return tmp;
}
template <class T>
T take(priority_queue<T> &O) {
  T tmp = O.top();
  O.pop();
  return tmp;
}
template <class T>
inline void getint(T &num) {
  bool neg = 0;
  num = 0;
  char c;
  while ((c = getchar()) && (!isdigit(c) && c != '-'))
    ;
  if (c == '-') {
    neg = 1;
    c = getchar();
  }
  do num = num * 10 + c - '0';
  while ((c = getchar()) && isdigit(c));
  num *= (neg) ? -1 : 1;
}
void OPEN(string in = "input.txt", string out = "output.txt") {
  freopen(in.c_str(), "r", stdin);
  freopen(out.c_str(), "w", stdout);
  return;
}
double dp[55][55][55], fact[55];
int n, p, arr[55], fixi, tot;
double ans;
double f(int pos, int nf, int tab) {
  if (pos == n) {
    if (tab + arr[fixi] > p)
      return nf * fact[nf] * fact[n - nf - 1];
    else
      return 0;
  }
  double &ret = dp[pos][nf][tab];
  if (ret == ret) return ret;
  if (pos == fixi) return ret = f(pos + 1, nf, tab);
  ret = 0;
  if (tab + arr[pos] <= p) ret += f(pos + 1, nf + 1, tab + arr[pos]);
  ret += f(pos + 1, nf, tab);
  return ret;
}
int main() {
  fact[0] = fact[1] = 1;
  for (int(i) = (2), _t = (51); i < (_t); ++(i)) fact[i] = fact[i - 1] * i;
  getint(n);
  for (int(i) = (0), _t = (n); i < (_t); ++(i)) {
    getint(arr[i]);
    tot += arr[i];
  }
  getint(p);
  if (tot <= p) {
    printf("%d\n", n);
    return 0;
  }
  for (int(i) = (0), _t = (n); i < (_t); ++(i)) {
    fixi = i;
    memset(dp, -1, sizeof(dp));
    ans += f(0, 0, 0);
  }
  {};
  printf("%.10lf\n", ans / fact[n]);
  return 0;
}
