#include <bits/stdc++.h>
using namespace std;
int main() {
  srand((unsigned int)time(NULL));
  int n;
  int a[55];
  int p;
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  cin >> p;
  double ret = 0.0;
  for (int i = 1; i <= n; i++) {
    long long dp[2505][55] = {};
    dp[0][0] = 1LL;
    for (int j = 1; j <= n; j++) {
      if (i == j) continue;
      for (int k = 2500; k >= 0; k--) {
        for (int b = j - 1; b >= 0; b--) {
          if (!dp[k][b]) continue;
          dp[k + a[j]][b + 1] += dp[k][b];
        }
      }
    }
    for (int j = 0; j <= p - a[i]; j++) {
      for (int k = 0; k < n; k++) {
        if (!dp[j][k]) continue;
        double x = (double)dp[j][k];
        double cur = 1.0;
        for (int s = 1; s <= k; s++) {
          x *= s;
          while (x > 1e10 && cur <= n) {
            x /= cur;
            cur += 1.0;
          }
        }
        for (int s = 1; s <= n - 1 - k; s++) {
          x *= s;
          while (x > 1e10 && cur <= n) {
            x /= cur;
            cur += 1.0;
          }
        }
        while (cur <= n) {
          x /= cur;
          cur += 1.0;
        }
        ret += x;
      }
    }
  }
  cout << ret << endl;
}
