#include <bits/stdc++.h>
using namespace std;
int n;
int a[101];
float 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 / (a[i] * a[i]);
  }
  ans = 1 / (num * num);
  cout << ans << endl;
  return 0;
}
