import java.io.*; 
import java.math.*;
import java.util.*;
 
public class Main {
	static final long MOD = 998244353;
	//static final long MOD = 1000000007;
	static boolean[] visited;
    public static void main(String[] args) throws IOException {
        FastScanner sc = new FastScanner();
        int N = sc.nextInt();
        double[] nums = new double[N];
        for (int i = 0; i < N; i++)
        	nums[i] = sc.nextDouble();
        double den = 0;
        for (double num: nums) {
        	den += 1/num;
        }
        System.out.println(1/den);
    }
    
    public static int[] sort(int[] array) {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array);

		return array;
	}
    
    public static int[][] sort(int[][] array) {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int[] temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array, new Comparator<int[]>() {
			  @Override
			  public int compare(int[] arr1, int[] arr2) {
				  //Ascending order.
				  return arr1[0]-arr2[0];
			  }
		});
		return array;
	}
    
    static class FastScanner { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastScanner() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine());
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try { 
                str = br.readLine(); 
            } catch (IOException e) {
                e.printStackTrace(); 
            } 
            return str; 
        }
    }
}

class Node {
	public HashSet<Node> children;
	public int n;
	
	public Node(int n) {
		this.n = n;
		children = new HashSet<Node>();
	}
	
	public void addChild(Node node) {
		children.add(node);
	}
	
	public void removeChild(Node node) {
		children.remove(node);
	}

	/* (non-Javadoc)
	 * @see java.lang.Object#hashCode()
	 */
	@Override
	public int hashCode() {
		return n;
	}

	@Override
	public boolean equals(Object obj) {
		if (! (obj instanceof Node)) {
			return false;
		} else {
			Node node = (Node) obj;
			return (n == node.n);
		}
	}
	
	public String toString() {
		return (this.n+1)+"";
	}
}

class BinaryIndexedTree {
	  public long[] arr;
 
	  public BinaryIndexedTree (int N) {
	    arr = new long[N+1];
	    arr[0] = 0;
	  }
 
	  //add k to the i-th element.
	  public void add(long k, int i) {
	    int node = i+1;
	    while (node < arr.length) {
	      arr[node] += k;
	      node += node & (-node);
	    }
	  }
 
	  //sum up the elements from input[s_i] to input[e_i], from [s_i,e_i).
	  public long sum(int s_i, int e_i) {
	    return sum(e_i) - sum(s_i);
	  }
 
	  public long sum(int i) {
	    long total = 0;
	    int node = i;
	    while (node > 0) {
	      total += arr[node];
	      node -= node & (-node);
	    }
	    return total;
	  }
}

class DSU {
	public int N;
	public int[] parent;
	public int[] rank;
	public int count;
	
	public DSU(int numNodes) {
		N = numNodes;
	    parent = new int[N];
	    rank = new int[N];
	    for (int i = 0; i < N; i++) {
	    	parent[i] = i;
	    	rank[i] = 1;
	    }
	    count = numNodes;
	}
	
	public boolean isConnected(int p, int q) {
	    return root(p) == root(q);
	}
	
	public int root(int p) {
		while (p != parent[p])
			p = parent[p];
	    return p;
	}
	
	//only connect p and q if they are disjointed.
	public void connect(int p, int q) {
		int rootP = root(p);
	    int rootQ = root(q);
	    if (rank[rootP] >= rank[rootQ]) {
	    	parent[rootQ] = rootP;
	    	rank[rootP] += rank[rootQ];
	    } else if (rank[rootQ] > rank[rootP]) {
	    	parent[rootP] = rootQ;
	    	rank[rootQ] += rank[rootP];
	    }
	    count--;
	}
} Comparator<int[]>() {
			  @Override
			  public int compare(int[] arr1, int[] arr2) {
				  //Ascending order.
				  return arr1[0]-arr2[0];
			  }
		} @Override
			  public int compare(int[] arr1, int[] arr2) {
				  //Ascending order.
				  return arr1[0]-arr2[0];
			  } compare {
				  //Ascending order.
				  return arr1[0]-arr2[0];
			  } return arr1[0]-arr2[0]; arr1[0]-arr2[0] arr1[0] arr1 0 arr2[0] arr2 0 int[] arr1 arr1 int[] arr2 arr2 import java.io.*; io.* java import java.math.*; math.* java import java.util.*; util.* java public class Main {
	static final long MOD = 998244353;
	//static final long MOD = 1000000007;
	static boolean[] visited;
    public static void main(String[] args) throws IOException {
        FastScanner sc = new FastScanner();
        int N = sc.nextInt();
        double[] nums = new double[N];
        for (int i = 0; i < N; i++)
        	nums[i] = sc.nextDouble();
        double den = 0;
        for (double num: nums) {
        	den += 1/num;
        }
        System.out.println(1/den);
    }
    
    public static int[] sort(int[] array) {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array);

		return array;
	}
    
    public static int[][] sort(int[][] array) {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int[] temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array, new Comparator<int[]>() {
			  @Override
			  public int compare(int[] arr1, int[] arr2) {
				  //Ascending order.
				  return arr1[0]-arr2[0];
			  }
		});
		return array;
	}
    
    static class FastScanner { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastScanner() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine());
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try { 
                str = br.readLine(); 
            } catch (IOException e) {
                e.printStackTrace(); 
            } 
            return str; 
        }
    }
} Main static final long MOD = 998244353; MOD 998244353 //static final long MOD = 1000000007;
	static boolean[] visited; visited public static void main(String[] args) throws IOException {
        FastScanner sc = new FastScanner();
        int N = sc.nextInt();
        double[] nums = new double[N];
        for (int i = 0; i < N; i++)
        	nums[i] = sc.nextDouble();
        double den = 0;
        for (double num: nums) {
        	den += 1/num;
        }
        System.out.println(1/den);
    } main {
        FastScanner sc = new FastScanner();
        int N = sc.nextInt();
        double[] nums = new double[N];
        for (int i = 0; i < N; i++)
        	nums[i] = sc.nextDouble();
        double den = 0;
        for (double num: nums) {
        	den += 1/num;
        }
        System.out.println(1/den);
    } FastScanner sc = new FastScanner(); sc new FastScanner() int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double[] nums = new double[N]; nums new double[N] N for (int i = 0; i < N; i++)
        	nums[i] = sc.nextDouble(); int i = 0; int i = 0; i 0 i < N i N i++ i++ i nums[i] = sc.nextDouble(); nums[i] = sc.nextDouble() nums[i] nums i sc.nextDouble() sc.nextDouble sc double den = 0; den 0 for (double num: nums) {
        	den += 1/num;
        } double num nums {
        	den += 1/num;
        } {
        	den += 1/num;
        } den += 1/num den 1/num 1 num System.out.println(1/den) System.out.println System.out System System.out 1/den 1 den String[] args args public static int[] sort(int[] array) {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array);

		return array;
	} sort {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array);

		return array;
	} Random rgen = new Random(); rgen new Random() for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		} int i = 0; int i = 0; i 0 i< array.length i array.length array array.length i++ i++ i {
		    int randomPosition = rgen.nextInt(array.length);
		    int temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		} {
		    int randomPosition = rgen.nextInt(array.length);
		    int temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		} int randomPosition = rgen.nextInt(array.length); randomPosition rgen.nextInt(array.length) rgen.nextInt rgen array.length array array.length int temp = array[i]; temp array[i] array i array[i] = array[randomPosition] array[i] array i array[randomPosition] array randomPosition array[randomPosition] = temp array[randomPosition] array randomPosition temp Arrays.sort(array) Arrays.sort Arrays array return array; array int[] array array public static int[][] sort(int[][] array) {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int[] temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array, new Comparator<int[]>() {
			  @Override
			  public int compare(int[] arr1, int[] arr2) {
				  //Ascending order.
				  return arr1[0]-arr2[0];
			  }
		});
		return array;
	} sort {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int[] temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array, new Comparator<int[]>() {
			  @Override
			  public int compare(int[] arr1, int[] arr2) {
				  //Ascending order.
				  return arr1[0]-arr2[0];
			  }
		});
		return array;
	} Random rgen = new Random(); rgen new Random() for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int[] temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		} int i = 0; int i = 0; i 0 i< array.length i array.length array array.length i++ i++ i {
		    int randomPosition = rgen.nextInt(array.length);
		    int[] temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		} {
		    int randomPosition = rgen.nextInt(array.length);
		    int[] temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		} int randomPosition = rgen.nextInt(array.length); randomPosition rgen.nextInt(array.length) rgen.nextInt rgen array.length array array.length int[] temp = array[i]; temp array[i] array i array[i] = array[randomPosition] array[i] array i array[randomPosition] array randomPosition array[randomPosition] = temp array[randomPosition] array randomPosition temp Arrays.sort(array, new Comparator<int[]>() {
			  @Override
			  public int compare(int[] arr1, int[] arr2) {
				  //Ascending order.
				  return arr1[0]-arr2[0];
			  }
		}) Arrays.sort Arrays array new Comparator<int[]>() {
			  @Override
			  public int compare(int[] arr1, int[] arr2) {
				  //Ascending order.
				  return arr1[0]-arr2[0];
			  }
		} return array; array int[][] array array static class FastScanner { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastScanner() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine());
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try { 
                str = br.readLine(); 
            } catch (IOException e) {
                e.printStackTrace(); 
            } 
            return str; 
        }
    } FastScanner BufferedReader br; br StringTokenizer st; st public FastScanner() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } FastScanner { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } br = new BufferedReader(new InputStreamReader(System.in)) br new BufferedReader(new InputStreamReader(System.in)) String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine());
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } next { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine());
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine());
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } st == null || !st.hasMoreElements() st == null st null !st.hasMoreElements() st.hasMoreElements() st.hasMoreElements st { 
                try { 
                    st = new StringTokenizer(br.readLine());
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } try { 
                    st = new StringTokenizer(br.readLine());
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } IOException  e { 
                    e.printStackTrace(); 
                } e.printStackTrace() e.printStackTrace e { 
                    st = new StringTokenizer(br.readLine());
                } st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return st.nextToken(); st.nextToken() st.nextToken st int nextInt() { 
            return Integer.parseInt(next()); 
        } nextInt { 
            return Integer.parseInt(next()); 
        } return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next long nextLong() { 
            return Long.parseLong(next()); 
        } nextLong { 
            return Long.parseLong(next()); 
        } return Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next double nextDouble() { 
            return Double.parseDouble(next()); 
        } nextDouble { 
            return Double.parseDouble(next()); 
        } return Double.parseDouble(next()); Double.parseDouble(next()) Double.parseDouble Double next() next String nextLine() { 
            String str = ""; 
            try { 
                str = br.readLine(); 
            } catch (IOException e) {
                e.printStackTrace(); 
            } 
            return str; 
        } nextLine { 
            String str = ""; 
            try { 
                str = br.readLine(); 
            } catch (IOException e) {
                e.printStackTrace(); 
            } 
            return str; 
        } String str = ""; str "" try { 
                str = br.readLine(); 
            } catch (IOException e) {
                e.printStackTrace(); 
            } catch (IOException e) {
                e.printStackTrace(); 
            } IOException e {
                e.printStackTrace(); 
            } e.printStackTrace() e.printStackTrace e { 
                str = br.readLine(); 
            } str = br.readLine() str br.readLine() br.readLine br return str; str class Node {
	public HashSet<Node> children;
	public int n;
	
	public Node(int n) {
		this.n = n;
		children = new HashSet<Node>();
	}
	
	public void addChild(Node node) {
		children.add(node);
	}
	
	public void removeChild(Node node) {
		children.remove(node);
	}

	/* (non-Javadoc)
	 * @see java.lang.Object#hashCode()
	 */
	@Override
	public int hashCode() {
		return n;
	}

	@Override
	public boolean equals(Object obj) {
		if (! (obj instanceof Node)) {
			return false;
		} else {
			Node node = (Node) obj;
			return (n == node.n);
		}
	}
	
	public String toString() {
		return (this.n+1)+"";
	}
} Node public HashSet<Node> children; children public int n; n public Node(int n) {
		this.n = n;
		children = new HashSet<Node>();
	} Node {
		this.n = n;
		children = new HashSet<Node>();
	} this.n = n this.n this this.n n children = new HashSet<Node>() children new HashSet<Node>() int n n public void addChild(Node node) {
		children.add(node);
	} addChild {
		children.add(node);
	} children.add(node) children.add children node Node node node public void removeChild(Node node) {
		children.remove(node);
	} removeChild {
		children.remove(node);
	} children.remove(node) children.remove children node Node node node /* (non-Javadoc)
	 * @see java.lang.Object#hashCode()
	 */
	@Override
	public int hashCode() {
		return n;
	} hashCode {
		return n;
	} return n; n @Override
	public boolean equals(Object obj) {
		if (! (obj instanceof Node)) {
			return false;
		} else {
			Node node = (Node) obj;
			return (n == node.n);
		}
	} equals {
		if (! (obj instanceof Node)) {
			return false;
		} else {
			Node node = (Node) obj;
			return (n == node.n);
		}
	} if (! (obj instanceof Node)) {
			return false;
		} else {
			Node node = (Node) obj;
			return (n == node.n);
		} ! (obj instanceof Node) (obj instanceof Node) obj Node {
			return false;
		} return false; false {
			Node node = (Node) obj;
			return (n == node.n);
		} Node node = (Node) obj; node (Node) obj return (n == node.n); (n == node.n) n node.n node node.n Object obj obj public String toString() {
		return (this.n+1)+"";
	} toString {
		return (this.n+1)+"";
	} return (this.n+1)+""; (this.n+1)+"" (this.n+1) this.n this this.n 1 "" class BinaryIndexedTree {
	  public long[] arr;
 
	  public BinaryIndexedTree (int N) {
	    arr = new long[N+1];
	    arr[0] = 0;
	  }
 
	  //add k to the i-th element.
	  public void add(long k, int i) {
	    int node = i+1;
	    while (node < arr.length) {
	      arr[node] += k;
	      node += node & (-node);
	    }
	  }
 
	  //sum up the elements from input[s_i] to input[e_i], from [s_i,e_i).
	  public long sum(int s_i, int e_i) {
	    return sum(e_i) - sum(s_i);
	  }
 
	  public long sum(int i) {
	    long total = 0;
	    int node = i;
	    while (node > 0) {
	      total += arr[node];
	      node -= node & (-node);
	    }
	    return total;
	  }
} BinaryIndexedTree public long[] arr; arr public BinaryIndexedTree (int N) {
	    arr = new long[N+1];
	    arr[0] = 0;
	  } BinaryIndexedTree {
	    arr = new long[N+1];
	    arr[0] = 0;
	  } arr = new long[N+1] arr new long[N+1] N+1 N 1 arr[0] = 0 arr[0] arr 0 0 int N N //add k to the i-th element.
	  public void add(long k, int i) {
	    int node = i+1;
	    while (node < arr.length) {
	      arr[node] += k;
	      node += node & (-node);
	    }
	  } add {
	    int node = i+1;
	    while (node < arr.length) {
	      arr[node] += k;
	      node += node & (-node);
	    }
	  } int node = i+1; node i+1 i 1 while (node < arr.length) {
	      arr[node] += k;
	      node += node & (-node);
	    } node < arr.length node arr.length arr arr.length {
	      arr[node] += k;
	      node += node & (-node);
	    } arr[node] += k arr[node] arr node k node += node & (-node) node node & (-node) node (-node) node long k k int i i //sum up the elements from input[s_i] to input[e_i], from [s_i,e_i).
	  public long sum(int s_i, int e_i) {
	    return sum(e_i) - sum(s_i);
	  } sum {
	    return sum(e_i) - sum(s_i);
	  } return sum(e_i) - sum(s_i); sum(e_i) - sum(s_i) sum(e_i) sum e_i sum(s_i) sum s_i int s_i s_i int e_i e_i public long sum(int i) {
	    long total = 0;
	    int node = i;
	    while (node > 0) {
	      total += arr[node];
	      node -= node & (-node);
	    }
	    return total;
	  } sum {
	    long total = 0;
	    int node = i;
	    while (node > 0) {
	      total += arr[node];
	      node -= node & (-node);
	    }
	    return total;
	  } long total = 0; total 0 int node = i; node i while (node > 0) {
	      total += arr[node];
	      node -= node & (-node);
	    } node > 0 node 0 {
	      total += arr[node];
	      node -= node & (-node);
	    } total += arr[node] total arr[node] arr node node -= node & (-node) node node & (-node) node (-node) node return total; total int i i class DSU {
	public int N;
	public int[] parent;
	public int[] rank;
	public int count;
	
	public DSU(int numNodes) {
		N = numNodes;
	    parent = new int[N];
	    rank = new int[N];
	    for (int i = 0; i < N; i++) {
	    	parent[i] = i;
	    	rank[i] = 1;
	    }
	    count = numNodes;
	}
	
	public boolean isConnected(int p, int q) {
	    return root(p) == root(q);
	}
	
	public int root(int p) {
		while (p != parent[p])
			p = parent[p];
	    return p;
	}
	
	//only connect p and q if they are disjointed.
	public void connect(int p, int q) {
		int rootP = root(p);
	    int rootQ = root(q);
	    if (rank[rootP] >= rank[rootQ]) {
	    	parent[rootQ] = rootP;
	    	rank[rootP] += rank[rootQ];
	    } else if (rank[rootQ] > rank[rootP]) {
	    	parent[rootP] = rootQ;
	    	rank[rootQ] += rank[rootP];
	    }
	    count--;
	}
} DSU public int N; N public int[] parent; parent public int[] rank; rank public int count; count public DSU(int numNodes) {
		N = numNodes;
	    parent = new int[N];
	    rank = new int[N];
	    for (int i = 0; i < N; i++) {
	    	parent[i] = i;
	    	rank[i] = 1;
	    }
	    count = numNodes;
	} DSU {
		N = numNodes;
	    parent = new int[N];
	    rank = new int[N];
	    for (int i = 0; i < N; i++) {
	    	parent[i] = i;
	    	rank[i] = 1;
	    }
	    count = numNodes;
	} N = numNodes N numNodes parent = new int[N] parent new int[N] N rank = new int[N] rank new int[N] N for (int i = 0; i < N; i++) {
	    	parent[i] = i;
	    	rank[i] = 1;
	    } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
	    	parent[i] = i;
	    	rank[i] = 1;
	    } {
	    	parent[i] = i;
	    	rank[i] = 1;
	    } parent[i] = i parent[i] parent i i rank[i] = 1 rank[i] rank i 1 count = numNodes count numNodes int numNodes numNodes public boolean isConnected(int p, int q) {
	    return root(p) == root(q);
	} isConnected {
	    return root(p) == root(q);
	} return root(p) == root(q); root(p) == root(q) root(p) root p root(q) root q int p p int q q public int root(int p) {
		while (p != parent[p])
			p = parent[p];
	    return p;
	} root {
		while (p != parent[p])
			p = parent[p];
	    return p;
	} while (p != parent[p])
			p = parent[p]; p != parent[p] p parent[p] parent p p = parent[p] p parent[p] parent p return p; p int p p //only connect p and q if they are disjointed.
	public void connect(int p, int q) {
		int rootP = root(p);
	    int rootQ = root(q);
	    if (rank[rootP] >= rank[rootQ]) {
	    	parent[rootQ] = rootP;
	    	rank[rootP] += rank[rootQ];
	    } else if (rank[rootQ] > rank[rootP]) {
	    	parent[rootP] = rootQ;
	    	rank[rootQ] += rank[rootP];
	    }
	    count--;
	} connect {
		int rootP = root(p);
	    int rootQ = root(q);
	    if (rank[rootP] >= rank[rootQ]) {
	    	parent[rootQ] = rootP;
	    	rank[rootP] += rank[rootQ];
	    } else if (rank[rootQ] > rank[rootP]) {
	    	parent[rootP] = rootQ;
	    	rank[rootQ] += rank[rootP];
	    }
	    count--;
	} int rootP = root(p); rootP root(p) root p int rootQ = root(q); rootQ root(q) root q if (rank[rootP] >= rank[rootQ]) {
	    	parent[rootQ] = rootP;
	    	rank[rootP] += rank[rootQ];
	    } else if (rank[rootQ] > rank[rootP]) {
	    	parent[rootP] = rootQ;
	    	rank[rootQ] += rank[rootP];
	    } rank[rootP] >= rank[rootQ] rank[rootP] rank rootP rank[rootQ] rank rootQ {
	    	parent[rootQ] = rootP;
	    	rank[rootP] += rank[rootQ];
	    } parent[rootQ] = rootP parent[rootQ] parent rootQ rootP rank[rootP] += rank[rootQ] rank[rootP] rank rootP rank[rootQ] rank rootQ if (rank[rootQ] > rank[rootP]) {
	    	parent[rootP] = rootQ;
	    	rank[rootQ] += rank[rootP];
	    } rank[rootQ] > rank[rootP] rank[rootQ] rank rootQ rank[rootP] rank rootP {
	    	parent[rootP] = rootQ;
	    	rank[rootQ] += rank[rootP];
	    } parent[rootP] = rootQ parent[rootP] parent rootP rootQ rank[rootQ] += rank[rootP] rank[rootQ] rank rootQ rank[rootP] rank rootP count-- count int p p int q q public class Main {
	static final long MOD = 998244353;
	//static final long MOD = 1000000007;
	static boolean[] visited;
    public static void main(String[] args) throws IOException {
        FastScanner sc = new FastScanner();
        int N = sc.nextInt();
        double[] nums = new double[N];
        for (int i = 0; i < N; i++)
        	nums[i] = sc.nextDouble();
        double den = 0;
        for (double num: nums) {
        	den += 1/num;
        }
        System.out.println(1/den);
    }
    
    public static int[] sort(int[] array) {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array);

		return array;
	}
    
    public static int[][] sort(int[][] array) {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int[] temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array, new Comparator<int[]>() {
			  @Override
			  public int compare(int[] arr1, int[] arr2) {
				  //Ascending order.
				  return arr1[0]-arr2[0];
			  }
		});
		return array;
	}
    
    static class FastScanner { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastScanner() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine());
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try { 
                str = br.readLine(); 
            } catch (IOException e) {
                e.printStackTrace(); 
            } 
            return str; 
        }
    }
} public class Main {
	static final long MOD = 998244353;
	//static final long MOD = 1000000007;
	static boolean[] visited;
    public static void main(String[] args) throws IOException {
        FastScanner sc = new FastScanner();
        int N = sc.nextInt();
        double[] nums = new double[N];
        for (int i = 0; i < N; i++)
        	nums[i] = sc.nextDouble();
        double den = 0;
        for (double num: nums) {
        	den += 1/num;
        }
        System.out.println(1/den);
    }
    
    public static int[] sort(int[] array) {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array);

		return array;
	}
    
    public static int[][] sort(int[][] array) {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int[] temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array, new Comparator<int[]>() {
			  @Override
			  public int compare(int[] arr1, int[] arr2) {
				  //Ascending order.
				  return arr1[0]-arr2[0];
			  }
		});
		return array;
	}
    
    static class FastScanner { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastScanner() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine());
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try { 
                str = br.readLine(); 
            } catch (IOException e) {
                e.printStackTrace(); 
            } 
            return str; 
        }
    }
} Main
import java.io.*; 
import java.math.*;
import java.util.*;
 
public class Main {
	static final long MOD = 998244353;
	//static final long MOD = 1000000007;
	static boolean[] visited;
    public static void main(String[] args) throws IOException {
        FastScanner sc = new FastScanner();
        int N = sc.nextInt();
        double[] nums = new double[N];
        for (int i = 0; i < N; i++)
        	nums[i] = sc.nextDouble();
        double den = 0;
        for (double num: nums) {
        	den += 1/num;
        }
        System.out.println(1/den);
    }
    
    public static int[] sort(int[] array) {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array);

		return array;
	}
    
    public static int[][] sort(int[][] array) {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i< array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int[] temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array, new Comparator<int[]>() {
			  @Override
			  public int compare(int[] arr1, int[] arr2) {
				  //Ascending order.
				  return arr1[0]-arr2[0];
			  }
		});
		return array;
	}
    
    static class FastScanner { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastScanner() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine());
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try { 
                str = br.readLine(); 
            } catch (IOException e) {
                e.printStackTrace(); 
            } 
            return str; 
        }
    }
}

class Node {
	public HashSet<Node> children;
	public int n;
	
	public Node(int n) {
		this.n = n;
		children = new HashSet<Node>();
	}
	
	public void addChild(Node node) {
		children.add(node);
	}
	
	public void removeChild(Node node) {
		children.remove(node);
	}

	/* (non-Javadoc)
	 * @see java.lang.Object#hashCode()
	 */
	@Override
	public int hashCode() {
		return n;
	}

	@Override
	public boolean equals(Object obj) {
		if (! (obj instanceof Node)) {
			return false;
		} else {
			Node node = (Node) obj;
			return (n == node.n);
		}
	}
	
	public String toString() {
		return (this.n+1)+"";
	}
}

class BinaryIndexedTree {
	  public long[] arr;
 
	  public BinaryIndexedTree (int N) {
	    arr = new long[N+1];
	    arr[0] = 0;
	  }
 
	  //add k to the i-th element.
	  public void add(long k, int i) {
	    int node = i+1;
	    while (node < arr.length) {
	      arr[node] += k;
	      node += node & (-node);
	    }
	  }
 
	  //sum up the elements from input[s_i] to input[e_i], from [s_i,e_i).
	  public long sum(int s_i, int e_i) {
	    return sum(e_i) - sum(s_i);
	  }
 
	  public long sum(int i) {
	    long total = 0;
	    int node = i;
	    while (node > 0) {
	      total += arr[node];
	      node -= node & (-node);
	    }
	    return total;
	  }
}

class DSU {
	public int N;
	public int[] parent;
	public int[] rank;
	public int count;
	
	public DSU(int numNodes) {
		N = numNodes;
	    parent = new int[N];
	    rank = new int[N];
	    for (int i = 0; i < N; i++) {
	    	parent[i] = i;
	    	rank[i] = 1;
	    }
	    count = numNodes;
	}
	
	public boolean isConnected(int p, int q) {
	    return root(p) == root(q);
	}
	
	public int root(int p) {
		while (p != parent[p])
			p = parent[p];
	    return p;
	}
	
	//only connect p and q if they are disjointed.
	public void connect(int p, int q) {
		int rootP = root(p);
	    int rootQ = root(q);
	    if (rank[rootP] >= rank[rootQ]) {
	    	parent[rootQ] = rootP;
	    	rank[rootP] += rank[rootQ];
	    } else if (rank[rootQ] > rank[rootP]) {
	    	parent[rootP] = rootQ;
	    	rank[rootQ] += rank[rootP];
	    }
	    count--;
	}
}