#include <bits/stdc++.h>
using namespace std;
long long n, a[52], p;
long double dp[52][52][52];
long double f(long long i, long long j, long long s) {
  if (i == 0 && j == 0 && s == 0) return 1.0;
  if (i == 0) return 0;
  long double &ret = dp[i][j][s];
  if (ret >= 0) return ret;
  long double prob = (long double)j / i;
  ret = (1.0 - prob) * f(i - 1, j, s);
  if (a[i] <= s) ret += prob * f(i - 1, j - 1, s - a[i]);
  return ret;
}
void solve() {
  cin >> n;
  for (long long i = 1; i <= n; i++) cin >> a[i];
  cin >> p;
  memset(dp, -1, sizeof(dp));
  long double ans = 0;
  for (long long i = 1; i <= n; i++) {
    for (long long j = 0; j <= p; j++) ans += f(n, i, j);
  }
  cout << fixed << setprecision(10) << ans << endl;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long t;
  t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
