#include <bits/stdc++.h>
using namespace std;
double fac[52], dp[52][52];
int n, p;
int a[52];
int main() {
  cin >> n;
  fac[0] = dp[0][0] = 1;
  for (int i = 1; i <= n; i++) {
    fac[i] = fac[i - 1] * i;
    cin >> a[i];
  }
  cin >> p;
  for (int i = 1; i <= n; i++)
    for (int j = i; j >= 1; j--)
      for (int k = a[i]; k <= p; k++) dp[j][k] += dp[j - 1][k - a[i]];
  double ans = 0;
  for (int i = 1; i <= n; i++)
    for (int j = 1; j <= p; j++) ans += dp[i][j] * fac[i] * fac[n - i];
  cout << ans / fac[n] << "\n";
  return 0;
}
