#include <bits/stdc++.h>
using namespace std;
int n, p;
int a[55];
long double dp[55][55][55];
long double fact[55];
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; ++i) scanf("%d", &a[i]);
  scanf("%d", &p);
  dp[0][0][0] = 1;
  for (int i = 1; i <= n; ++i) {
    for (int j = 0; j <= i; ++j) {
      for (int k = 0; k <= p; ++k) {
        dp[i][j][k] = dp[i - 1][j][k];
        if (k - a[i] >= 0 && j > 0) dp[i][j][k] += dp[i - 1][j - 1][k - a[i]];
      }
    }
  }
  fact[0] = 1;
  for (int i = 1; i <= n; ++i) fact[i] = fact[i - 1] * i;
  long double Sol = 0;
  for (int j = 1; j <= n; ++j) {
    long double sum = 0;
    for (int k = 0; k <= p; ++k) sum += dp[n][j][k];
    Sol = Sol + (sum / fact[n] * fact[j] * fact[n - j]);
  }
  cout << Sol;
  return 0;
}
