
import java.io.*;
import java.util.StringTokenizer;
public class Main{
    public static void main(String[] args) throws IOException {
        scanner sc = new scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int n = sc.nextInt();
        double sum = 0;
        for (int i = 0; i < n; i++) {
            sum+= 1/sc.nextDouble();
        }
        out.print(1/sum);

        out.flush();

    }
}
class scanner{
    StringTokenizer st;
    BufferedReader br;
    public scanner(InputStream s){
        br = new BufferedReader(new InputStreamReader(s));
    }
    public scanner (String s) throws FileNotFoundException { br = new BufferedReader(new FileReader(s));}

    public String next() throws IOException {
        while (st == null || !st.hasMoreTokens())
            st = new StringTokenizer(br.readLine());
        return st.nextToken();
    }

    public int nextInt() throws IOException {
        return Integer.parseInt(next());
    }

    public long nextLong() throws IOException {
        return  Long.parseLong(next());
    }

    public String nextLine() throws IOException {
        return br.readLine();
    }

    public double nextDouble() throws IOException {
        return Double.parseDouble(next());
    }

    public boolean ready() throws IOException {
        return br.ready();
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz