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 num = lcm(a,n);
		//System.out.println(l);
		long deno = sum(a,n,num);
		//System.out.println(deno);
		double ans = num / (double)deno;
		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 n){
		long lc = 1;
		for(int i = 0; i < n; i++){
			lc =  (lc * a[i])/gcd(lc,a[i]);
		}
		return lc;
	}
	
	public static long sum(long[] a,int n,long num){
		long count = 0;
		for(int i = 0; i < n; i++){
			count += (num/a[i]);
		}
		return count;
	}
	
	
	
	} 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 num = lcm(a,n);
		//System.out.println(l);
		long deno = sum(a,n,num);
		//System.out.println(deno);
		double ans = num / (double)deno;
		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 n){
		long lc = 1;
		for(int i = 0; i < n; i++){
			lc =  (lc * a[i])/gcd(lc,a[i]);
		}
		return lc;
	}
	
	public static long sum(long[] a,int n,long num){
		long count = 0;
		for(int i = 0; i < n; i++){
			count += (num/a[i]);
		}
		return count;
	}
	
	
	
	} 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 num = lcm(a,n);
		//System.out.println(l);
		long deno = sum(a,n,num);
		//System.out.println(deno);
		double ans = num / (double)deno;
		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 num = lcm(a,n);
		//System.out.println(l);
		long deno = sum(a,n,num);
		//System.out.println(deno);
		double ans = num / (double)deno;
		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 num = lcm(a,n); num lcm(a,n) lcm a n long deno = sum(a,n,num); deno sum(a,n,num) sum a n num double ans = num / (double)deno; ans num / (double)deno num (double)deno 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 n){
		long lc = 1;
		for(int i = 0; i < n; i++){
			lc =  (lc * a[i])/gcd(lc,a[i]);
		}
		return lc;
	} lcm {
		long lc = 1;
		for(int i = 0; i < n; i++){
			lc =  (lc * a[i])/gcd(lc,a[i]);
		}
		return lc;
	} long lc = 1; lc 1 for(int i = 0; i < n; i++){
			lc =  (lc * a[i])/gcd(lc,a[i]);
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			lc =  (lc * a[i])/gcd(lc,a[i]);
		} {
			lc =  (lc * a[i])/gcd(lc,a[i]);
		} lc =  (lc * a[i])/gcd(lc,a[i]) lc (lc * a[i])/gcd(lc,a[i]) (lc * a[i]) lc a[i] a i gcd(lc,a[i]) gcd lc a[i] a i return lc; lc long[] a a long n n public static long sum(long[] a,int n,long num){
		long count = 0;
		for(int i = 0; i < n; i++){
			count += (num/a[i]);
		}
		return count;
	} sum {
		long count = 0;
		for(int i = 0; i < n; i++){
			count += (num/a[i]);
		}
		return count;
	} long count = 0; count 0 for(int i = 0; i < n; i++){
			count += (num/a[i]);
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			count += (num/a[i]);
		} {
			count += (num/a[i]);
		} count += (num/a[i]) count (num/a[i]) num a[i] a i return count; count long[] a a int n n long num num
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 num = lcm(a,n);
		//System.out.println(l);
		long deno = sum(a,n,num);
		//System.out.println(deno);
		double ans = num / (double)deno;
		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 n){
		long lc = 1;
		for(int i = 0; i < n; i++){
			lc =  (lc * a[i])/gcd(lc,a[i]);
		}
		return lc;
	}
	
	public static long sum(long[] a,int n,long num){
		long count = 0;
		for(int i = 0; i < n; i++){
			count += (num/a[i]);
		}
		return count;
	}
	
	
	
	}