#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int n;
  cin >> n;
  long long int a[n];
  long long int sum = 0, pro = 1;
  long double res;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    pro *= a[i];
  }
  long long int k = pro;
  for (int i = 0; i < n; i++) {
    sum += k / a[i];
  }
  res = (long double)pro / (long double)sum;
  cout << res << endl;
  return 0;
}
