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

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = Integer.parseInt(sc.next());
        BigDecimal sum = BigDecimal.ZERO;
        for (int i = 0; i < N; i++) {
            BigDecimal n = new BigDecimal(sc.next());
            n = BigDecimal.ONE.divide(n, 7, RoundingMode.HALF_EVEN);
            sum = sum.add(n, MathContext.UNLIMITED);
        }

        System.out.println(BigDecimal.ONE.divide(sum, 6, RoundingMode.HALF_EVEN).toPlainString());
        sc.close();
    }
}
/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