#include <bits/stdc++.h>
using namespace std;
template <typename T, typename U>
inline void smin(T &a, const U &b) {
  if (a > b) a = b;
}
template <typename T, typename U>
inline void smax(T &a, const U &b) {
  if (a < b) a = b;
}
template <typename T>
inline void gn(T &first) {
  char c, sg = 0;
  while (c = getchar(), (c > '9' || c < '0') && c != '-')
    ;
  for ((c == '-' ? sg = 1, c = getchar() : 0), first = 0; c >= '0' && c <= '9';
       c = getchar())
    first = (first << 1) + (first << 3) + c - '0';
  if (sg) first = -first;
}
template <class T, class T1>
inline void gn(T &first, T1 &second) {
  gn(first);
  gn(second);
}
template <class T, class T1, class T2>
inline void gn(T &first, T1 &second, T2 &z) {
  gn(first);
  gn(second);
  gn(z);
}
template <typename T>
inline void print(T first) {
  if (first < 0) {
    putchar('-');
    return print(-first);
  }
  if (first < 10) {
    putchar('0' + first);
    return;
  }
  print(first / 10);
  putchar(first % 10 + '0');
}
template <typename T>
inline void println(T first) {
  print(first), putchar('\n');
}
template <typename T>
inline void printsp(T first) {
  print(first), putchar(' ');
}
template <class T, class T1>
inline void print(T first, T1 second) {
  printsp(first), println(second);
}
template <class T, class T1, class T2>
inline void print(T first, T1 second, T2 z) {
  printsp(first), printsp(second), println(z);
}
int power(int a, int b, int m, int ans = 1) {
  for (; b; b >>= 1, a = (long long)a * a % m)
    if (b & 1) ans = (long long)ans * a % m;
  return ans;
}
int a[55];
long double f[55], ans;
long double dp[55][55][55];
int n, p, sum;
void calc() {
  for (int i = 0; i <= n; i++) {
    for (int j = 0; j <= i; j++) {
      for (int k = 0; k <= p; k++) {
        dp[i][j][k] = 0;
      }
    }
  }
  dp[0][0][0] = 1;
  for (int i = 1; i < n; i++) {
    for (int j = 0; j < i; j++) {
      for (int k = 0; k <= p; k++) {
        dp[i][j][k] = dp[i - 1][j][k];
      }
    }
    for (int j = i; j--;) {
      for (int k = p; k--;)
        if (k + a[i] <= p) {
          dp[i][j + 1][k + a[i]] += dp[i][j][k];
        }
    }
  }
  for (int i = 1; i < n; i++) {
    for (int j = 1; j <= p; j++) {
      ans += dp[n - 1][i][j] * f[i] * f[n - 1 - i];
    }
  }
}
int main() {
  f[0] = 1;
  for (int i = 1; i < 55; i++) f[i] = f[i - 1] * i;
  gn(n);
  for (int i = 1; i <= n; i++) {
    gn(a[i]);
    sum += a[i];
  }
  gn(p);
  if (sum <= p) {
    println(n);
    return 0;
  }
  for (int i = 1; i <= n; i++) {
    swap(a[i], a[n]);
    calc();
    swap(a[i], a[n]);
  }
  ans /= f[n];
  cout << fixed << setprecision(10);
  cout << ans << endl;
}
