#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  double S;
  S = 0;
  cin >> N;
  vector<int> A(N);
  for (int i = 0; i < N; i++) {
    cin >> A[i];
    S += 1 / A[i];
  }
  double ans;
  ans = 1 / S;
  cout << ans << endl;
  return 0;
}
