#include <bits/stdc++.h>
using namespace std;
double dp[55][55];
int n, p, a[55];
double fi[55];
void init() {
  fi[0] = 1.0;
  for (int i = 1; i < 55; i++) fi[i] = i * fi[i - 1];
}
int main() {
  init();
  double ans = 0;
  dp[0][0] = 1;
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
  scanf("%d", &p);
  for (int i = 1; i <= n; i++)
    for (int j = n; j >= 1; j--)
      for (int t = p; t >= a[i]; t--) dp[j][t] += dp[j - 1][t - a[i]];
  for (int i = 1; i <= n; i++)
    for (int j = 1; j <= p; j++) ans += dp[i][j] * fi[i] * fi[n - i];
  printf("%.6lf\n", ans / fi[n]);
  return 0;
}
