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