#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  int N;
  vector<int> a(N);
  for (int i = 0; i < N; i++) {
    cin >> a.at(i);
  }
  double answer = 0.00000;
  for (int i = 0; i < N; i++) {
    answer += double(1 / a.at(i));
  }
  cout << double(1 / answer) << endl;
}
