import java.util.*;

class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		long[] a = new long[n];
		for(int i = 0; i < n; i++){
			a[i] = sc.nextLong();
		}
		long deno = 1;
		for(long in: a) deno = lcm(in, deno);

		long num = 0;
		for(long in: a) num += deno / in;
		//System.out.println(deno);
		double ans = (double)deno / num;
		System.out.println(ans);
	
	}
	
	public static long gcd(long a, long b){
		if (a == 0) return b;
		return gcd(b % a, a);
	}
	
	public static long lcm(long a, long b){
		return a * b / gcd(a, b);
	}
} import java.util.*; util.* java class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		long[] a = new long[n];
		for(int i = 0; i < n; i++){
			a[i] = sc.nextLong();
		}
		long deno = 1;
		for(long in: a) deno = lcm(in, deno);

		long num = 0;
		for(long in: a) num += deno / in;
		//System.out.println(deno);
		double ans = (double)deno / num;
		System.out.println(ans);
	
	}
	
	public static long gcd(long a, long b){
		if (a == 0) return b;
		return gcd(b % a, a);
	}
	
	public static long lcm(long a, long b){
		return a * b / gcd(a, b);
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		long[] a = new long[n];
		for(int i = 0; i < n; i++){
			a[i] = sc.nextLong();
		}
		long deno = 1;
		for(long in: a) deno = lcm(in, deno);

		long num = 0;
		for(long in: a) num += deno / in;
		//System.out.println(deno);
		double ans = (double)deno / num;
		System.out.println(ans);
	
	} main {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		long[] a = new long[n];
		for(int i = 0; i < n; i++){
			a[i] = sc.nextLong();
		}
		long deno = 1;
		for(long in: a) deno = lcm(in, deno);

		long num = 0;
		for(long in: a) num += deno / in;
		//System.out.println(deno);
		double ans = (double)deno / num;
		System.out.println(ans);
	
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc long[] a = new long[n]; a new long[n] n for(int i = 0; i < n; i++){
			a[i] = sc.nextLong();
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			a[i] = sc.nextLong();
		} {
			a[i] = sc.nextLong();
		} a[i] = sc.nextLong() a[i] a i sc.nextLong() sc.nextLong sc long deno = 1; deno 1 for(long in: a) deno = lcm(in, deno); long in a deno = lcm(in, deno); deno = lcm(in, deno) deno lcm(in, deno) lcm in deno long num = 0; num 0 for(long in: a) num += deno / in; long in a num += deno / in; num += deno / in num deno / in deno in double ans = (double)deno / num; ans (double)deno / num (double)deno num System.out.println(ans) System.out.println System.out System System.out ans String[] args args public static long gcd(long a, long b){
		if (a == 0) return b;
		return gcd(b % a, a);
	} gcd {
		if (a == 0) return b;
		return gcd(b % a, a);
	} if (a == 0) return b; a == 0 a 0 return b; b return gcd(b % a, a); gcd(b % a, a) gcd b % a b a a long a a long b b public static long lcm(long a, long b){
		return a * b / gcd(a, b);
	} lcm {
		return a * b / gcd(a, b);
	} return a * b / gcd(a, b); a * b / gcd(a, b) a * b a b gcd(a, b) gcd a b long a a long b b
import java.util.*;

class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		long[] a = new long[n];
		for(int i = 0; i < n; i++){
			a[i] = sc.nextLong();
		}
		long deno = 1;
		for(long in: a) deno = lcm(in, deno);

		long num = 0;
		for(long in: a) num += deno / in;
		//System.out.println(deno);
		double ans = (double)deno / num;
		System.out.println(ans);
	
	}
	
	public static long gcd(long a, long b){
		if (a == 0) return b;
		return gcd(b % a, a);
	}
	
	public static long lcm(long a, long b){
		return a * b / gcd(a, b);
	}
}
