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

int main(void){
    int N,*a;
    double s=0;
    scanf("%d",&N);
    a = malloc(sizeof(int)*N);
    for(int i=0;i<N;i++){
        scanf("%d",&a[i]);
        s +=(double) 1/a[i];
    }
    printf("%lf",1/s);
    return 0;
}