#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  long long a[n];
  long long bunshi = 1;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    bunshi *= a[i];
  }
  long long bunbo = 0;
  for (int i = 0; i < n; i++) {
    bunbo += bunshi / a[i];
  }
  cout << setprecision(30) << (double)bunshi / (double)bunbo << endl;
  return 0;
}
