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