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