#include <bits/stdc++.h>
using namespace std;
const int nmax = 52;
long double dp[nmax][nmax][nmax];
int n, p;
int a[nmax];
long double fact[52];
double solve() {
  dp[0][0][0] = 1.0;
  for (int i = 1; i <= n; i++) {
    dp[i][0][0] = 1.0;
    for (int k = 1; k <= i; k++) {
      for (int j = 1; j <= p; j++) {
        dp[i][k][j] = dp[i - 1][k][j];
        if (j - a[i] >= 0) {
          dp[i][k][j] += dp[i - 1][k - 1][j - a[i]];
        }
      }
    }
  }
  fact[0] = 1.0;
  for (int i = 1; i <= n; i++) {
    fact[i] = fact[i - 1] * i;
  }
  long double ans = 0;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= p; j++) {
      ans += dp[n][i][j] * fact[n - i] * i * fact[i - 1];
    }
  }
  return ans / fact[n];
}
void readData() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  cin >> p;
  sort(a + 1, a + n + 1);
}
int main() {
  readData();
  cout.precision(10);
  cout << fixed << solve();
  return 0;
}
