#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  if (n == 1) {
    int x;
    cin >> x;
    cout << x << endl;
    return 0;
  }
  long double mult = 1;
  long double sum = 0;
  vector<int> v;
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    v.push_back(x);
    mult *= x;
    sum += x;
  }
  long double fsum = 0;
  for (int i = 0; i < n; i++) {
    fsum += sum - v[i];
  }
  long double ans = mult / fsum;
  cout << ans << endl;
}
