#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  vector<int> A(N);
  for (int i = 0; i < N; i++) {
    cin >> A.at(i);
  }
  double S = 0;
  for (int i = 0; i < N; i++) {
    S += (1.0 / A[i]);
  }
  double T = 0;
  T = 1.0 / S;
  cout << T << endl;
}
