import java.util.Scanner;

/**
 * B - Resistors in Parallel
 * N 個の整数の列A1,…,AN が与えられます。
 * これらの逆数の総和の逆数を求めてください。
 */
public class Main {

    public static void main(String[] args) {

        Scanner sc = new Scanner( System.in );
        // 整数の数
        int n = sc.nextInt();
//        System.out.println("整数：" +n);
        double result = 0;
        // 整数の列
        double[] a = new double[n];
        for( int i = 0; i < n; i++ ) {
            a[i] = sc.nextDouble();
            result += 1 / a[i];
        }
        System.out.println( 1 / result );
    }
} import java.util.Scanner; Scanner java.util public class Main {

    public static void main(String[] args) {

        Scanner sc = new Scanner( System.in );
        // 整数の数
        int n = sc.nextInt();
//        System.out.println("整数：" +n);
        double result = 0;
        // 整数の列
        double[] a = new double[n];
        for( int i = 0; i < n; i++ ) {
            a[i] = sc.nextDouble();
            result += 1 / a[i];
        }
        System.out.println( 1 / result );
    }
} Main public static void main(String[] args) {

        Scanner sc = new Scanner( System.in );
        // 整数の数
        int n = sc.nextInt();
//        System.out.println("整数：" +n);
        double result = 0;
        // 整数の列
        double[] a = new double[n];
        for( int i = 0; i < n; i++ ) {
            a[i] = sc.nextDouble();
            result += 1 / a[i];
        }
        System.out.println( 1 / result );
    } main {

        Scanner sc = new Scanner( System.in );
        // 整数の数
        int n = sc.nextInt();
//        System.out.println("整数：" +n);
        double result = 0;
        // 整数の列
        double[] a = new double[n];
        for( int i = 0; i < n; i++ ) {
            a[i] = sc.nextDouble();
            result += 1 / a[i];
        }
        System.out.println( 1 / result );
    } Scanner sc = new Scanner( System.in ); sc new Scanner( System.in ) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double result = 0; result 0 double[] a = new double[n]; a new double[n] n for( int i = 0; i < n; i++ ) {
            a[i] = sc.nextDouble();
            result += 1 / a[i];
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            a[i] = sc.nextDouble();
            result += 1 / a[i];
        } {
            a[i] = sc.nextDouble();
            result += 1 / a[i];
        } a[i] = sc.nextDouble() a[i] a i sc.nextDouble() sc.nextDouble sc result += 1 / a[i] result 1 / a[i] 1 a[i] a i System.out.println( 1 / result ) System.out.println System.out System System.out 1 / result 1 result String[] args args public class Main {

    public static void main(String[] args) {

        Scanner sc = new Scanner( System.in );
        // 整数の数
        int n = sc.nextInt();
//        System.out.println("整数：" +n);
        double result = 0;
        // 整数の列
        double[] a = new double[n];
        for( int i = 0; i < n; i++ ) {
            a[i] = sc.nextDouble();
            result += 1 / a[i];
        }
        System.out.println( 1 / result );
    }
} public class Main {

    public static void main(String[] args) {

        Scanner sc = new Scanner( System.in );
        // 整数の数
        int n = sc.nextInt();
//        System.out.println("整数：" +n);
        double result = 0;
        // 整数の列
        double[] a = new double[n];
        for( int i = 0; i < n; i++ ) {
            a[i] = sc.nextDouble();
            result += 1 / a[i];
        }
        System.out.println( 1 / result );
    }
} Main
import java.util.Scanner;

/**
 * B - Resistors in Parallel
 * N 個の整数の列A1,…,AN が与えられます。
 * これらの逆数の総和の逆数を求めてください。
 */
public class Main {

    public static void main(String[] args) {

        Scanner sc = new Scanner( System.in );
        // 整数の数
        int n = sc.nextInt();
//        System.out.println("整数：" +n);
        double result = 0;
        // 整数の列
        double[] a = new double[n];
        for( int i = 0; i < n; i++ ) {
            a[i] = sc.nextDouble();
            result += 1 / a[i];
        }
        System.out.println( 1 / result );
    }
}