#include <iostream>
#include <string>
#include <vector>
using namespace std;

int main() {
  int n, x;
  string temp;
  while ((cin >> n) && (n != 0)) {
    vector<double> prob(1001, 0);
    prob[0] = 1;
    for (int i = 0; i < n; i++) {
      cin >> temp;
      int m = atoi(temp.substr(1).c_str());
      vector<double> new_prob(1001, 0);
      for (int j = 0; j <= 1000; j++)
      for (int k = 1; k <= m; k++)
      if (j >= k)
        new_prob[j] += prob[j-k] * 1.0 / m;
      prob = new_prob;
    }
    cin >> x;
    cout.precision(5);
    cout << fixed << prob[x] << endl;
  }
  return 0;
}