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());
        double[] A = new double[N];
        for (int i = 0; i < N; i++) {
            A[i] = Double.parseDouble(sc.next());
        }

        double d = 0;
        for (int i = 0; i < N; i++) {
            d += 1d / A[i];
        }
        System.out.println(1d / d);
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = Integer.parseInt(sc.next());
        double[] A = new double[N];
        for (int i = 0; i < N; i++) {
            A[i] = Double.parseDouble(sc.next());
        }

        double d = 0;
        for (int i = 0; i < N; i++) {
            d += 1d / A[i];
        }
        System.out.println(1d / d);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = Integer.parseInt(sc.next());
        double[] A = new double[N];
        for (int i = 0; i < N; i++) {
            A[i] = Double.parseDouble(sc.next());
        }

        double d = 0;
        for (int i = 0; i < N; i++) {
            d += 1d / A[i];
        }
        System.out.println(1d / d);
    } main {
        Scanner sc = new Scanner(System.in);
        int N = Integer.parseInt(sc.next());
        double[] A = new double[N];
        for (int i = 0; i < N; i++) {
            A[i] = Double.parseDouble(sc.next());
        }

        double d = 0;
        for (int i = 0; i < N; i++) {
            d += 1d / A[i];
        }
        System.out.println(1d / d);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = Integer.parseInt(sc.next()); N Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc double[] A = new double[N]; A new double[N] N for (int i = 0; i < N; i++) {
            A[i] = Double.parseDouble(sc.next());
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            A[i] = Double.parseDouble(sc.next());
        } {
            A[i] = Double.parseDouble(sc.next());
        } A[i] = Double.parseDouble(sc.next()) A[i] A i Double.parseDouble(sc.next()) Double.parseDouble Double sc.next() sc.next sc double d = 0; d 0 for (int i = 0; i < N; i++) {
            d += 1d / A[i];
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            d += 1d / A[i];
        } {
            d += 1d / A[i];
        } d += 1d / A[i] d 1d / A[i] 1d A[i] A i System.out.println(1d / d) System.out.println System.out System System.out 1d / d 1d d String[] args args public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = Integer.parseInt(sc.next());
        double[] A = new double[N];
        for (int i = 0; i < N; i++) {
            A[i] = Double.parseDouble(sc.next());
        }

        double d = 0;
        for (int i = 0; i < N; i++) {
            d += 1d / A[i];
        }
        System.out.println(1d / d);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = Integer.parseInt(sc.next());
        double[] A = new double[N];
        for (int i = 0; i < N; i++) {
            A[i] = Double.parseDouble(sc.next());
        }

        double d = 0;
        for (int i = 0; i < N; i++) {
            d += 1d / A[i];
        }
        System.out.println(1d / d);
    }
} Main
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());
        double[] A = new double[N];
        for (int i = 0; i < N; i++) {
            A[i] = Double.parseDouble(sc.next());
        }

        double d = 0;
        for (int i = 0; i < N; i++) {
            d += 1d / A[i];
        }
        System.out.println(1d / d);
    }
}
