#include <bits/stdc++.h>
using namespace std;
int n, a[55], p;
double fac[55], f[55][55], tol, ans;
int main() {
  scanf("%d", &n);
  fac[0] = 1;
  for (int i = 1; i <= n; i++) {
    scanf("%d", a + i);
    fac[i] = fac[i - 1] * i;
    tol += a[i];
  }
  scanf("%d", &p);
  if (tol <= p) {
    printf("%d", n);
    return 0;
  }
  f[0][0] = 1;
  for (int i = 1; i <= n; i++)
    for (int j = n - 1; j >= 0; j--)
      for (int k = 0; k + a[i] <= p; k++) f[j + 1][k + a[i]] += f[j][k];
  for (int j = 1; j <= n; j++)
    for (int k = 0; k <= p; k++) ans += f[j][k] * fac[j] * fac[n - j];
  ans /= fac[n];
  printf("%f", ans);
}
