#include <bits/stdc++.h>
int main() {
  int n, p, i, j, k, l;
  double sum = 0;
  int a[50];
  long long b[51][51];
  scanf("%d", &n);
  for (i = 0; i < n; i++) scanf("%d", &a[i]);
  scanf("%d", &p);
  for (i = 0; i < n; i++) {
    double tmp = 0;
    for (j = 0; j <= n; j++) {
      for (k = 0; k <= p; k++) {
        b[j][k] = 0;
      }
    }
    b[0][0] = 1;
    for (j = 0; j < n; j++) {
      if (j == i) continue;
      for (k = n - 1; k >= 0; k--) {
        for (l = p; l >= 0; l--) {
          if (b[k][l] == 0) continue;
          if (l + a[j] <= p) b[k + 1][l + a[j]] += b[k][l];
        }
      }
    }
    for (j = 0; j <= n; j++) {
      for (k = 0; k <= p - a[i]; k++) {
        double tmp2 = b[j][k];
        for (l = 1; l <= j; l++) tmp2 *= l;
        for (l = 1; l <= n - j - 1; l++) tmp2 *= l;
        tmp += tmp2;
      }
    }
    for (j = 1; j <= n; j++) tmp /= j;
    sum += tmp;
  }
  printf("%lf\n", sum);
  return 0;
}
