#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  double a, sum = 0, ans;
  for (int i = 0; i < n; i++) {
    cin >> a;
    sum += 1 / a;
  }
  ans = 1 / sum;
  cout << fixed << setprecision(10) << ans << endl;
}
