#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int& i : a) cin >> i;
  double ans = 0;
  for (int i : a) ans += 1.0 / i;
  ans = 1.0 / ans;
  cout << ans << endl;
  return 0;
}
