#include <bits/stdc++.h>
int i, c, p, w, n, a[52];
double f[52], dp[52][52], r;
int main() {
  f[0] = dp[0][0] = 1;
  scanf("%d", &n);
  for (i = 1; i <= n; scanf("%d", &a[i++])) f[i] = f[i - 1] * i;
  scanf("%d", &p);
  for (i = 1; i <= n; i++)
    for (c = i; c > 0; c--)
      for (w = a[i]; w <= p; w++) dp[c][w] += dp[c - 1][w - a[i]];
  for (c = 1; c <= n; c++)
    for (w = 1; w <= p; w++) r += dp[c][w] * f[c] * f[n - c];
  printf("%.5f\n", r / f[n]);
}
