#include <bits/stdc++.h>
using namespace std;
int i, n, m, j, a[120], k;
double p[120], ans;
long long f[120][120];
int main() {
  scanf("%d", &n);
  for (i = 1; i <= n; ++i) scanf("%d", &a[i]);
  scanf("%d", &m);
  f[0][0] = 1;
  for (i = 1; i <= n; ++i)
    for (j = n; j >= 0; --j)
      for (k = m; k >= a[i]; --k) f[j][k] += f[j - 1][k - a[i]];
  p[0] = 1;
  for (i = 1; i <= n; ++i) p[i] = p[i - 1] * i;
  for (i = 0; i <= n; ++i)
    for (j = 1; j <= m; ++j) ans += f[i][j] * p[i] * p[n - i];
  printf("%.10lf\n", ans / p[n]);
}
