#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  int a[100];
  double rev = 0.0;
  double ans = 0.0;
  cin >> N;
  for (int i = 1; i <= N; i++) {
    cin >> a[i];
    ans += 1.0 / a[i];
  }
  ans = 1.0 / ans;
  cout << ans;
  return 0;
}
