import java.math.BigDecimal;
import java.math.BigInteger;
import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		BigInteger m = BigInteger.ONE;
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextInt();
			m = m.multiply(BigInteger.valueOf(a[i]));
		}
		BigInteger de = BigInteger.ZERO;
		for (int i = 0; i < n; i++) {
			de = de.add(m.divide(BigInteger.valueOf(a[i])));
		}
		System.out.println(new BigDecimal(m).divide(new BigDecimal(de), 10, BigDecimal.ROUND_HALF_UP));
	}
} import java.math.BigDecimal; BigDecimal java.math import java.math.BigInteger; BigInteger java.math import java.util.*; util.* java public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		BigInteger m = BigInteger.ONE;
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextInt();
			m = m.multiply(BigInteger.valueOf(a[i]));
		}
		BigInteger de = BigInteger.ZERO;
		for (int i = 0; i < n; i++) {
			de = de.add(m.divide(BigInteger.valueOf(a[i])));
		}
		System.out.println(new BigDecimal(m).divide(new BigDecimal(de), 10, BigDecimal.ROUND_HALF_UP));
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		BigInteger m = BigInteger.ONE;
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextInt();
			m = m.multiply(BigInteger.valueOf(a[i]));
		}
		BigInteger de = BigInteger.ZERO;
		for (int i = 0; i < n; i++) {
			de = de.add(m.divide(BigInteger.valueOf(a[i])));
		}
		System.out.println(new BigDecimal(m).divide(new BigDecimal(de), 10, BigDecimal.ROUND_HALF_UP));
	} main {
		Scanner sc = new Scanner(System.in);
		BigInteger m = BigInteger.ONE;
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextInt();
			m = m.multiply(BigInteger.valueOf(a[i]));
		}
		BigInteger de = BigInteger.ZERO;
		for (int i = 0; i < n; i++) {
			de = de.add(m.divide(BigInteger.valueOf(a[i])));
		}
		System.out.println(new BigDecimal(m).divide(new BigDecimal(de), 10, BigDecimal.ROUND_HALF_UP));
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) BigInteger m = BigInteger.ONE; m BigInteger.ONE BigInteger BigInteger.ONE 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();
			m = m.multiply(BigInteger.valueOf(a[i]));
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			a[i] = sc.nextInt();
			m = m.multiply(BigInteger.valueOf(a[i]));
		} {
			a[i] = sc.nextInt();
			m = m.multiply(BigInteger.valueOf(a[i]));
		} a[i] = sc.nextInt() a[i] a i sc.nextInt() sc.nextInt sc m = m.multiply(BigInteger.valueOf(a[i])) m m.multiply(BigInteger.valueOf(a[i])) m.multiply m BigInteger.valueOf(a[i]) BigInteger.valueOf BigInteger a[i] a i BigInteger de = BigInteger.ZERO; de BigInteger.ZERO BigInteger BigInteger.ZERO for (int i = 0; i < n; i++) {
			de = de.add(m.divide(BigInteger.valueOf(a[i])));
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			de = de.add(m.divide(BigInteger.valueOf(a[i])));
		} {
			de = de.add(m.divide(BigInteger.valueOf(a[i])));
		} de = de.add(m.divide(BigInteger.valueOf(a[i]))) de de.add(m.divide(BigInteger.valueOf(a[i]))) de.add de m.divide(BigInteger.valueOf(a[i])) m.divide m BigInteger.valueOf(a[i]) BigInteger.valueOf BigInteger a[i] a i System.out.println(new BigDecimal(m).divide(new BigDecimal(de), 10, BigDecimal.ROUND_HALF_UP)) System.out.println System.out System System.out new BigDecimal(m).divide(new BigDecimal(de), 10, BigDecimal.ROUND_HALF_UP) new BigDecimal(m).divide new BigDecimal(m) new BigDecimal(de) 10 BigDecimal.ROUND_HALF_UP BigDecimal BigDecimal.ROUND_HALF_UP String[] args args public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		BigInteger m = BigInteger.ONE;
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextInt();
			m = m.multiply(BigInteger.valueOf(a[i]));
		}
		BigInteger de = BigInteger.ZERO;
		for (int i = 0; i < n; i++) {
			de = de.add(m.divide(BigInteger.valueOf(a[i])));
		}
		System.out.println(new BigDecimal(m).divide(new BigDecimal(de), 10, BigDecimal.ROUND_HALF_UP));
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		BigInteger m = BigInteger.ONE;
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextInt();
			m = m.multiply(BigInteger.valueOf(a[i]));
		}
		BigInteger de = BigInteger.ZERO;
		for (int i = 0; i < n; i++) {
			de = de.add(m.divide(BigInteger.valueOf(a[i])));
		}
		System.out.println(new BigDecimal(m).divide(new BigDecimal(de), 10, BigDecimal.ROUND_HALF_UP));
	}
} Main
import java.math.BigDecimal;
import java.math.BigInteger;
import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		BigInteger m = BigInteger.ONE;
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextInt();
			m = m.multiply(BigInteger.valueOf(a[i]));
		}
		BigInteger de = BigInteger.ZERO;
		for (int i = 0; i < n; i++) {
			de = de.add(m.divide(BigInteger.valueOf(a[i])));
		}
		System.out.println(new BigDecimal(m).divide(new BigDecimal(de), 10, BigDecimal.ROUND_HALF_UP));
	}
}
