#include <bits/stdc++.h>
using namespace std;
long long i, j, k, x, y, z, n, m;
long long f[51][51];
long long a[51];
double ans;
void mem() { memset(f, 0, sizeof(f)); }
int main() {
  scanf("%d", &n);
  for (i = 1; i <= n; i++) scanf("%d", &a[i]);
  for (i = 1, x = 0; i <= n; i++) x += a[i];
  scanf("%d", &m);
  if (x <= m) {
    printf("%d\n", n);
    return 0;
  }
  for (i = 1; i <= n; i++) {
    mem();
    f[0][0] = 1;
    for (j = 1; j <= n; j++)
      if (i != j)
        for (k = m; k >= a[j]; k--)
          for (x = n; x > 0; x--) f[k][x] += f[k - a[j]][x - 1];
    for (x = 1; x <= n; x++) {
      long long s = 0;
      for (j = max(m - a[i] + 1, (long long)0); j <= m; j++) s += f[j][x];
      double p = s;
      for (j = x + 1; j <= n; j++) p /= ((double)j);
      for (j = 1; j <= n - x - 1; j++) p *= ((double)j);
      p *= x;
      ans += p;
    }
  }
  printf("%.8f\n", ans);
  return 0;
}
