#include <bits/stdc++.h>
int main(void) {
  int N, A[110], i;
  double a, b;
  scanf("%d", &N);
  for (i = 0; i < N; i++) scanf("%d", &A[i]);
  for (i = 0; i < N; i++) a += (double)1 / A[i];
  b = 1 / a;
  printf("%.5lf\n", b);
  return 0;
}
