#include <bits/stdc++.h>
using namespace std;
const int inf = 1 << 28;
int a[55];
double dp[55][105], f[55][105][55];
double A(int n) {
  if (n < 0) return 0;
  double ans = 1;
  for (int i = 2; i <= n; i++) {
    ans *= i;
  }
  return ans;
}
int cmp(int x, int y) { return x > y; }
int main() {
  int n, p;
  scanf("%d", &n);
  int sum = 0;
  for (int i = 0; i < n; i++) {
    scanf("%d", &a[i]);
    sum += a[i];
  }
  scanf("%d", &p);
  if (sum <= p) {
    printf("%.10lf\n", (double)n);
    return 0;
  }
  dp[0][0] = 1;
  double ans = 0;
  for (int i = 0; i < n; i++) {
    for (int j = n; j > 0; j--) {
      for (int k = 0; k <= p; k++) {
        if (k >= a[i]) dp[j][k] += dp[j - 1][k - a[i]];
      }
    }
  }
  for (int i = 0; i < n; i++) f[0][0][i] = 1;
  for (int i = 1; i <= n; i++)
    for (int j = 0; j <= p; j++)
      for (int k = 0; k < n; k++) {
        if (j >= a[k])
          f[i][j][k] = dp[i][j] - f[i - 1][j - a[k]][k];
        else
          f[i][j][k] = dp[i][j];
      }
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j <= p; j++) {
      for (int k = 0; k < n; k++) {
        if (j + a[k] > p) {
          ans += A(i) * f[i][j][k] * A(n - i - 1) * i;
        }
      }
    }
  }
  printf("%.10lf\n", ans / A(n));
}
