import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int[] A = new int[N];
		double [] B = new double[N];
		double S =0;
		for(int i=0; i<N; i++) {
			A[i]=sc.nextInt();
		}
		for(int i=0; i<N; i++) {
          	B[i]=1.0/A[i];
			S=S+B[i];
		}
		System.out.println(1.0/S);
	}
}


/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz