#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  double s = 0.0, A;
  for (int i = 0; i < N; ++i) {
    cin >> A;
    s += 1.0 / A;
  }
  double ans = 1.0 / s;
  cout << ans << endl;
}
