#include <bits/stdc++.h>
using namespace std;
signed main() {
  long long int n;
  cin >> n;
  vector<long long int> a(n);
  for (long long int i = 0; i < n; i++) cin >> a[i];
  double sum = 0;
  for (long long int i = 0; i < n; i++) {
    sum += 1.0 / (double)a[i];
  }
  double ans = 1.0 / sum;
  cout << setprecision(15) << ans << endl;
  return 0;
}
