import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;

class Main {
    public static void main(String[] args){
        try{
            BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
            int t=Integer.parseInt(br.readLine());
            double pi=3.14159;
            System.out.println(2*pi*t);


        }catch(Exception e){
            System.out.println("KKKK "+e.getMessage());
        }
    }
    static boolean check(int[] arr){
        int odd=0;
        for(int i=0;i<arr.length;i++){
            if(arr[i]%2!=0){
                odd++;
            }
        }
        if(odd<=1){
            return true;
        }
        return false;
    }
} import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io import java.util.ArrayList; ArrayList java.util import java.util.HashMap; HashMap java.util import java.util.HashSet; HashSet java.util import java.util.List; List java.util class Main {
    public static void main(String[] args){
        try{
            BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
            int t=Integer.parseInt(br.readLine());
            double pi=3.14159;
            System.out.println(2*pi*t);


        }catch(Exception e){
            System.out.println("KKKK "+e.getMessage());
        }
    }
    static boolean check(int[] arr){
        int odd=0;
        for(int i=0;i<arr.length;i++){
            if(arr[i]%2!=0){
                odd++;
            }
        }
        if(odd<=1){
            return true;
        }
        return false;
    }
} Main public static void main(String[] args){
        try{
            BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
            int t=Integer.parseInt(br.readLine());
            double pi=3.14159;
            System.out.println(2*pi*t);


        }catch(Exception e){
            System.out.println("KKKK "+e.getMessage());
        }
    } main {
        try{
            BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
            int t=Integer.parseInt(br.readLine());
            double pi=3.14159;
            System.out.println(2*pi*t);


        }catch(Exception e){
            System.out.println("KKKK "+e.getMessage());
        }
    } try{
            BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
            int t=Integer.parseInt(br.readLine());
            double pi=3.14159;
            System.out.println(2*pi*t);


        }catch(Exception e){
            System.out.println("KKKK "+e.getMessage());
        } catch(Exception e){
            System.out.println("KKKK "+e.getMessage());
        } Exception e {
            System.out.println("KKKK "+e.getMessage());
        } System.out.println("KKKK "+e.getMessage()) System.out.println System.out System System.out "KKKK "+e.getMessage() "KKKK " e.getMessage() e.getMessage e {
            BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
            int t=Integer.parseInt(br.readLine());
            double pi=3.14159;
            System.out.println(2*pi*t);


        } BufferedReader br=new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) int t=Integer.parseInt(br.readLine()); t Integer.parseInt(br.readLine()) Integer.parseInt Integer br.readLine() br.readLine br double pi=3.14159; pi 3.14159 System.out.println(2*pi*t) System.out.println System.out System System.out 2*pi*t 2*pi*t 2 pi t String[] args args static boolean check(int[] arr){
        int odd=0;
        for(int i=0;i<arr.length;i++){
            if(arr[i]%2!=0){
                odd++;
            }
        }
        if(odd<=1){
            return true;
        }
        return false;
    } check {
        int odd=0;
        for(int i=0;i<arr.length;i++){
            if(arr[i]%2!=0){
                odd++;
            }
        }
        if(odd<=1){
            return true;
        }
        return false;
    } int odd=0; odd 0 for(int i=0;i<arr.length;i++){
            if(arr[i]%2!=0){
                odd++;
            }
        } int i=0; int i=0; i 0 i<arr.length i arr.length arr arr.length i++ i++ i {
            if(arr[i]%2!=0){
                odd++;
            }
        } {
            if(arr[i]%2!=0){
                odd++;
            }
        } if(arr[i]%2!=0){
                odd++;
            } arr[i]%2!=0 arr[i]%2 arr[i] arr i 2 0 {
                odd++;
            } odd++ odd if(odd<=1){
            return true;
        } odd<=1 odd 1 {
            return true;
        } return true; true return false; false int[] arr arr
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;

class Main {
    public static void main(String[] args){
        try{
            BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
            int t=Integer.parseInt(br.readLine());
            double pi=3.14159;
            System.out.println(2*pi*t);


        }catch(Exception e){
            System.out.println("KKKK "+e.getMessage());
        }
    }
    static boolean check(int[] arr){
        int odd=0;
        for(int i=0;i<arr.length;i++){
            if(arr[i]%2!=0){
                odd++;
            }
        }
        if(odd<=1){
            return true;
        }
        return false;
    }
}