#include <bits/stdc++.h>
int main(void) {
  int N, i;
  double A[1001];
  double sum = 0;
  scanf("%lf", &N);
  for (i = 0; i <= N; i++) {
    scanf("%lf", &A[i]);
  }
  for (i = 0; i <= N; i++) {
    sum = 1 / A[i] + sum;
  }
  printf("%lf", 1 / sum);
  return 0;
}
