#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  int a[110];
  for (int i = 0; i < N; i++) cin >> a[i];
  double d = 0;
  for (int i = 0; i < N; i++) d += 1 / a[i];
  double answer = 1 / d;
  cout << answer << endl;
  return 0;
}
