#include <stdio.h>
#include <stdlib.h>

int main(void){
  int size;
  double *seq;
  double child=0;
  scanf("%d",&size);

  if((seq=malloc(size*sizeof(double)))==NULL){
    printf("error");
    exit(1);
  }

  for(int i=0;i<size;i++){
    scanf("%lf",&seq[i]);
    child+=(1.0/seq[i]);
  }

  printf("%f\n",1.0/child);

  free((void *)seq);

  return 0;
}
