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

public class Main {
	public static void main(String args[]){

		Scanner sc = new Scanner(System.in);

		BigDecimal result = new BigDecimal("1");
		int n = sc.nextInt();

		double sum = 0;

		for(int i=0;i < n;i++) {
			BigDecimal next = new BigDecimal(sc.nextInt());
			BigDecimal div = BigDecimal.ONE.divide(next,10,BigDecimal.ROUND_HALF_UP);
			sum += div.doubleValue();
		}

		System.out.println(BigDecimal.ONE.divide(new BigDecimal(sum),20,BigDecimal.ROUND_HALF_UP));

	}
}
/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