#include <bits/stdc++.h>
using namespace std;
int main(int argc, char const *argv[]) {
  int N, A;
  double sum = 0;
  cin >> N;
  for (int i = 0; i < N; i++) {
    cin >> A;
    sum += 1.0 / double(A);
  }
  cout << 1.0 / sum << endl;
  return 0;
}
