#include <bits/stdc++.h>
using namespace std;
const long long N = 60;
const long long M = 20;
const long long SQ = 320;
const long long INF = 1e16;
const long long MOD = 1e9 + 7;
long long n, a[N], p;
long long dp[N][N][N];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  cin >> p;
  for (int i = 0; i <= p; i++) dp[0][0][i] = 1;
  for (int i = 1; i <= n; i++)
    for (int j = 0; j <= n; j++)
      for (int k = 0; k <= p; k++) {
        dp[i][j][k] = dp[i - 1][j][k];
        if (j && k - a[i] >= 0) dp[i][j][k] += dp[i - 1][j - 1][k - a[i]];
      }
  long double ans = 0;
  for (int i = 1; i <= n; i++) {
    long double t = dp[n][i][p];
    long double j = i, k = n;
    while (j || k > n - i) {
      if (j) t = t * j;
      if (k > n - i) t = t / k;
      j--;
      k--;
    }
    ans = ans + t;
  }
  cout << setprecision(15) << ans << '\n';
  return 0;
}
