import java.math.BigDecimal;
import java.util.Scanner;

public class Main {
    public static void main(String args[] ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        BigDecimal sum = new BigDecimal(0);
        for (int i = 0; i < n; i++) {
        	int num = sc.nextInt();
        	new BigDecimal(num);
        	BigDecimal gyaku =  new BigDecimal(1).divide(new BigDecimal(num), 10, BigDecimal.ROUND_DOWN);
        	sum = sum.add(gyaku);
		}
        System.out.println(new BigDecimal(1).divide(sum, 10, BigDecimal.ROUND_DOWN).toString());
    }
}

/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