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