#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, N, A[100];
  float sum = 0;
  cin >> N;
  for (i = 0; i < N; i++) {
    cin >> A[i];
    sum = sum + (1 / A[i]);
  }
  cout << 1 / sum;
  return 0;
}
