#include<stdio.h>
int main(void) {
	int N,i, A[100] = { 0 };
  double B=0, C=0;
	scanf("%d", &N);
	for(i=0;i<N;i++){
	 scanf("%d",&A[i]);
    }
	i = 0;
	while(i<N){
		if(A[i]=!0){
		B =B+1/A[i];
        }
		i++;
	}
	C = 1 / B;
	printf("%d", C);