#include <bits/stdc++.h>
using namespace std;
int main() {
  double n, m, ans = 0;
  cin >> n;
  for (int i = 0; i < (int)(n); i++) {
    cin >> m;
    m = 1 / m;
    ans += m;
  }
  cout << fixed << setprecision(8);
  ans = 1 / ans;
  cout << ans;
}
