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