import java.io.*;
import java.lang.*;
import java.util.*;
public class Main{
    public static void main(String[] args) throws IOException{
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int N=Integer.parseInt(br.readLine());
        int M=N*N*N;
        double ans=M/27.0;
        while(Math.abs(ans*27-M)>1e-6){
            double diff=Math.abs(ans*27-M);
            if(M>27*ans) ans=ans+diff/2;
            else ans=ans-diff/2;
        }
        System.out.println(ans);
    }
} import java.io.*; io.* java import java.lang.*; lang.* java import java.util.*; util.* java public class Main{
    public static void main(String[] args) throws IOException{
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int N=Integer.parseInt(br.readLine());
        int M=N*N*N;
        double ans=M/27.0;
        while(Math.abs(ans*27-M)>1e-6){
            double diff=Math.abs(ans*27-M);
            if(M>27*ans) ans=ans+diff/2;
            else ans=ans-diff/2;
        }
        System.out.println(ans);
    }
} Main public static void main(String[] args) throws IOException{
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int N=Integer.parseInt(br.readLine());
        int M=N*N*N;
        double ans=M/27.0;
        while(Math.abs(ans*27-M)>1e-6){
            double diff=Math.abs(ans*27-M);
            if(M>27*ans) ans=ans+diff/2;
            else ans=ans-diff/2;
        }
        System.out.println(ans);
    } main {
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int N=Integer.parseInt(br.readLine());
        int M=N*N*N;
        double ans=M/27.0;
        while(Math.abs(ans*27-M)>1e-6){
            double diff=Math.abs(ans*27-M);
            if(M>27*ans) ans=ans+diff/2;
            else ans=ans-diff/2;
        }
        System.out.println(ans);
    } BufferedReader br=new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) int N=Integer.parseInt(br.readLine()); N Integer.parseInt(br.readLine()) Integer.parseInt Integer br.readLine() br.readLine br int M=N*N*N; M N*N*N N*N*N N N N double ans=M/27.0; ans M/27.0 M 27.0 while(Math.abs(ans*27-M)>1e-6){
            double diff=Math.abs(ans*27-M);
            if(M>27*ans) ans=ans+diff/2;
            else ans=ans-diff/2;
        } Math.abs(ans*27-M)>1e-6 Math.abs(ans*27-M) Math.abs Math ans*27-M ans*27 ans 27 M 1e-6 {
            double diff=Math.abs(ans*27-M);
            if(M>27*ans) ans=ans+diff/2;
            else ans=ans-diff/2;
        } double diff=Math.abs(ans*27-M); diff Math.abs(ans*27-M) Math.abs Math ans*27-M ans*27 ans 27 M if(M>27*ans) ans=ans+diff/2;
            else ans=ans-diff/2; M>27*ans M 27*ans 27 ans ans=ans+diff/2 ans ans+diff/2 ans diff/2 diff 2 ans=ans-diff/2 ans ans-diff/2 ans diff/2 diff 2 System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main{
    public static void main(String[] args) throws IOException{
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int N=Integer.parseInt(br.readLine());
        int M=N*N*N;
        double ans=M/27.0;
        while(Math.abs(ans*27-M)>1e-6){
            double diff=Math.abs(ans*27-M);
            if(M>27*ans) ans=ans+diff/2;
            else ans=ans-diff/2;
        }
        System.out.println(ans);
    }
} public class Main{
    public static void main(String[] args) throws IOException{
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int N=Integer.parseInt(br.readLine());
        int M=N*N*N;
        double ans=M/27.0;
        while(Math.abs(ans*27-M)>1e-6){
            double diff=Math.abs(ans*27-M);
            if(M>27*ans) ans=ans+diff/2;
            else ans=ans-diff/2;
        }
        System.out.println(ans);
    }
} Main
import java.io.*;
import java.lang.*;
import java.util.*;
public class Main{
    public static void main(String[] args) throws IOException{
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int N=Integer.parseInt(br.readLine());
        int M=N*N*N;
        double ans=M/27.0;
        while(Math.abs(ans*27-M)>1e-6){
            double diff=Math.abs(ans*27-M);
            if(M>27*ans) ans=ans+diff/2;
            else ans=ans-diff/2;
        }
        System.out.println(ans);
    }
}