#include <bits/stdc++.h>
using namespace std;
const int mod = 1000000007;
long long int a, b, c, d, e, f, g, h[100005], arr[100005];
string s;
vector<long long int> v;
char ch;
int main() {
  cin >> a;
  double x;
  for (long long int i = 1; i <= a; i++) {
    cin >> arr[i];
    x += (double)1 / arr[i];
  }
  printf("%.6f", (double)1 / x);
}
