#include <bits/stdc++.h>
using namespace std;
int n, a[60], p, total;
double res, dp[60][60];
int main() {
  scanf("%d", &n);
  for (int i = (int)0; i <= (int)n - 1; ++i) scanf("%d", a + i), total += a[i];
  scanf("%d", &p);
  if (total <= p) {
    printf("%d", n);
    return 0;
  }
  dp[0][0] = 1.0;
  for (int i = (int)0; i <= (int)n - 1; ++i) {
    int cur = a[i];
    for (int k = (int)n - 1; k >= (int)0; --k)
      for (int j = (int)0; j <= (int)p - cur + 1 - 1; ++j)
        dp[k + 1][j + cur] += dp[k][j] / (n - k) * (k + 1);
  }
  for (int i = (int)1; i <= (int)n; ++i)
    for (int j = (int)0; j <= (int)p + 1 - 1; ++j) res += dp[i][j];
  printf("%.9lf", res);
}
