import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int a[] = new int[n];
        for (int i = 0; i < n; i++) {
            a[i] = sc.nextInt();
        }
        double seki = 1;
        long wa = 0;
        double b[] = new double[n];
        double wawa = 0;
        for (int i = 0; i < n; i++) {
            b[i] = 1.0 / (1.0 * a[i]);
            seki *= 1.0 * a[i];
            wawa += b[i];
        }
        for (int i = 0; i < n; i++) {
            wa += seki / a[i];
        }

        System.out.println(1.0 / wawa);
        sc.close();
    }
} import java.util.Scanner; Scanner java.util class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int a[] = new int[n];
        for (int i = 0; i < n; i++) {
            a[i] = sc.nextInt();
        }
        double seki = 1;
        long wa = 0;
        double b[] = new double[n];
        double wawa = 0;
        for (int i = 0; i < n; i++) {
            b[i] = 1.0 / (1.0 * a[i]);
            seki *= 1.0 * a[i];
            wawa += b[i];
        }
        for (int i = 0; i < n; i++) {
            wa += seki / a[i];
        }

        System.out.println(1.0 / wawa);
        sc.close();
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int a[] = new int[n];
        for (int i = 0; i < n; i++) {
            a[i] = sc.nextInt();
        }
        double seki = 1;
        long wa = 0;
        double b[] = new double[n];
        double wawa = 0;
        for (int i = 0; i < n; i++) {
            b[i] = 1.0 / (1.0 * a[i]);
            seki *= 1.0 * a[i];
            wawa += b[i];
        }
        for (int i = 0; i < n; i++) {
            wa += seki / a[i];
        }

        System.out.println(1.0 / wawa);
        sc.close();
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int a[] = new int[n];
        for (int i = 0; i < n; i++) {
            a[i] = sc.nextInt();
        }
        double seki = 1;
        long wa = 0;
        double b[] = new double[n];
        double wawa = 0;
        for (int i = 0; i < n; i++) {
            b[i] = 1.0 / (1.0 * a[i]);
            seki *= 1.0 * a[i];
            wawa += b[i];
        }
        for (int i = 0; i < n; i++) {
            wa += seki / a[i];
        }

        System.out.println(1.0 / wawa);
        sc.close();
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int a[] = new int[n]; a new int[n] n for (int i = 0; i < n; i++) {
            a[i] = sc.nextInt();
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            a[i] = sc.nextInt();
        } {
            a[i] = sc.nextInt();
        } a[i] = sc.nextInt() a[i] a i sc.nextInt() sc.nextInt sc double seki = 1; seki 1 long wa = 0; wa 0 double b[] = new double[n]; b new double[n] n double wawa = 0; wawa 0 for (int i = 0; i < n; i++) {
            b[i] = 1.0 / (1.0 * a[i]);
            seki *= 1.0 * a[i];
            wawa += b[i];
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            b[i] = 1.0 / (1.0 * a[i]);
            seki *= 1.0 * a[i];
            wawa += b[i];
        } {
            b[i] = 1.0 / (1.0 * a[i]);
            seki *= 1.0 * a[i];
            wawa += b[i];
        } b[i] = 1.0 / (1.0 * a[i]) b[i] b i 1.0 / (1.0 * a[i]) 1.0 (1.0 * a[i]) 1.0 a[i] a i seki *= 1.0 * a[i] seki 1.0 * a[i] 1.0 a[i] a i wawa += b[i] wawa b[i] b i for (int i = 0; i < n; i++) {
            wa += seki / a[i];
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            wa += seki / a[i];
        } {
            wa += seki / a[i];
        } wa += seki / a[i] wa seki / a[i] seki a[i] a i System.out.println(1.0 / wawa) System.out.println System.out System System.out 1.0 / wawa 1.0 wawa sc.close() sc.close sc String[] args args
import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int a[] = new int[n];
        for (int i = 0; i < n; i++) {
            a[i] = sc.nextInt();
        }
        double seki = 1;
        long wa = 0;
        double b[] = new double[n];
        double wawa = 0;
        for (int i = 0; i < n; i++) {
            b[i] = 1.0 / (1.0 * a[i]);
            seki *= 1.0 * a[i];
            wawa += b[i];
        }
        for (int i = 0; i < n; i++) {
            wa += seki / a[i];
        }

        System.out.println(1.0 / wawa);
        sc.close();
    }
}
