#include <bits/stdc++.h>
using namespace std;
int main() {
  int A[100], N;
  float AA = 0;
  cin >> N;
  for (int i = 0; i < N; i++) {
    cin >> A[i];
    AA += 1 / A[i];
  }
  AA = 1 / AA;
  cout << AA;
  return 0;
}
