#include <bits/stdc++.h>
using namespace std;
int a[55];
long long dp[55][55];
double pp[55];
int main() {
  int i, j, k, n, p;
  scanf("%d", &n);
  for (i = 1; i <= n; i++) scanf("%d", &a[i]);
  scanf("%d", &p);
  dp[0][0] = 1;
  for (i = 1; i <= n; i++)
    for (j = n; j >= 1; j--)
      for (k = p; k >= a[i]; k--) {
        dp[j][k] += dp[j - 1][k - a[i]];
      }
  pp[0] = 1;
  for (i = 1; i <= n; i++) pp[i] = pp[i - 1] * i;
  double ans = 0;
  for (i = 0; i <= n; i++) {
    for (j = 1; j <= p; j++) {
      ans += pp[i] * pp[n - i] * dp[i][j];
    }
  }
  printf("%.8lf\n", ans / pp[n]);
  return 0;
}
