#include <bits/stdc++.h>
using namespace std;
namespace IO {
const int MAXIOSIZE = 1 << 24 | 1;
unsigned char buf[MAXIOSIZE], *p1, *p2;
template <typename T>
void read(T& x) {
  x = 0;
  char ch =
      (p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, 1 << 24, stdin), p1 == p2)
           ? EOF
           : *p1++);
  bool flg = false;
  for (; ch < '0' || '9' < ch;
       ch = (p1 == p2 &&
                     (p2 = (p1 = buf) + fread(buf, 1, 1 << 24, stdin), p1 == p2)
                 ? EOF
                 : *p1++))
    if (ch == '-') flg |= true;
  for (; '0' <= ch && ch <= '9';
       ch = (p1 == p2 &&
                     (p2 = (p1 = buf) + fread(buf, 1, 1 << 24, stdin), p1 == p2)
                 ? EOF
                 : *p1++))
    x = x * 10 + ch - '0';
  flg ? x = -x : 0;
}
template <typename T>
void out(const T& x) {
  if (x > 9) out(x / 10);
  putchar(x % 10 + '0');
}
template <typename T>
inline void write(const T& x, const char& ed = ' ') {
  if (x < 0)
    putchar('-'), out(-x);
  else
    out(x);
  putchar(ed);
}
}  // namespace IO
const int MAXN = 50 + 10;
int n, val[MAXN], P;
long double prod[MAXN], f[MAXN][MAXN][MAXN * MAXN], ans, sum;
long double calc(int x) {
  memset(f, 0, sizeof f);
  f[0][0][0] = 1;
  long double res = 0;
  for (int i = (0), iend = (n - 1); i <= iend; ++i) {
    for (int j = (0), jend = (i); j <= jend; ++j) {
      for (int S = (0), Send = (P); S <= Send; ++S) {
        f[i + 1][j][S] += f[i][j][S];
        if (i + 1 != x) {
          f[i + 1][j + 1][S + val[i + 1]] += f[i][j][S];
        }
      }
    }
  }
  for (int cnt = (0), cntend = (n - 1); cnt <= cntend; ++cnt) {
    for (int S = (max(0, P - val[x] + 1)), Send = (P); S <= Send; ++S) {
      res += cnt * f[n][cnt][S] * prod[cnt] * prod[n - cnt - 1];
    }
  }
  return res;
}
int main() {
  IO::read(n);
  for (int i = (1), iend = (n); i <= iend; ++i) IO::read(val[i]), sum += val[i];
  IO::read(P);
  if (sum <= P) return printf("%d\n", n), 0;
  prod[0] = 1;
  for (int i = (1), iend = (n); i <= iend; ++i) prod[i] = prod[i - 1] * i;
  for (int i = (1), iend = (n); i <= iend; ++i) ans += calc(i);
  printf("%.10Lf\n", ans / prod[n]);
  return 0;
}
