#include <iostream>
#include <vector>
#include <cmath>

using namespace std;

typedef vector<int> VI;
typedef long double LD;
typedef long long int LL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;

LD ComputeProbability(const VI &sides, const int x) {
  VVLL sum(2, VLL(20 * sides.size() + 1));
  sum[0][0] = 1;

  if (x < 0) return 0;
  if (x >= sum[0].size()) return 0;

  // consider one die at a time
  
  LL den = 1;
  for (size_t i = 0; i < sides.size(); i++) {
    fill(sum[(i+1)%2].begin(), sum[(i+1)%2].end(), 0);
    den *= sides[i];

    // consider all possible rolls and new sums
    
    for (int roll = 1; roll <= sides[i]; roll++) {
      for (size_t j = roll; j < sum[0].size(); j++) {
	sum[(i+1)%2][j] += sum[i%2][j-roll];
      }
    }
  }

  // cannot overflow (den <= 20^13)

  return LD(sum[sides.size()%2][x]) / LD(den);
}

int main() {
  while (1) {
    int d;
    cin >> d;
    if (d == 0) break;

    VI sides;
    for (int i = 0; i < d; i++) {
      string s;
      cin >> s;
      sides.push_back(atoi(s.substr(1).c_str()));
    }
    
    int x;
    cin >> x;
    printf("%.5Lf\n", ComputeProbability(sides, x));
  }

  return 0;
}