import java.util.*;

public class Main {
    public static void main(String[] args) {
        Main main = new Main();
        main.run();
    }
    
    // 変数一覧
    double r;

    public void run() {
        // 入力情報取得
        Scanner sc = new Scanner(System.in);
        r = sc.nextInt();
        sc.close();

        // 解答処理
        solve();
    }
    
    private void solve() {
        double c = 2 * r * Math.PI;
        System.out.println(c);
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) {
        Main main = new Main();
        main.run();
    }
    
    // 変数一覧
    double r;

    public void run() {
        // 入力情報取得
        Scanner sc = new Scanner(System.in);
        r = sc.nextInt();
        sc.close();

        // 解答処理
        solve();
    }
    
    private void solve() {
        double c = 2 * r * Math.PI;
        System.out.println(c);
    }
} Main // 変数一覧
    double r; r public static void main(String[] args) {
        Main main = new Main();
        main.run();
    } main {
        Main main = new Main();
        main.run();
    } Main main = new Main(); main new Main() main.run() main.run main String[] args args public void run() {
        // 入力情報取得
        Scanner sc = new Scanner(System.in);
        r = sc.nextInt();
        sc.close();

        // 解答処理
        solve();
    } run {
        // 入力情報取得
        Scanner sc = new Scanner(System.in);
        r = sc.nextInt();
        sc.close();

        // 解答処理
        solve();
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) r = sc.nextInt() r sc.nextInt() sc.nextInt sc sc.close() sc.close sc solve() solve private void solve() {
        double c = 2 * r * Math.PI;
        System.out.println(c);
    } solve {
        double c = 2 * r * Math.PI;
        System.out.println(c);
    } double c = 2 * r * Math.PI; c 2 * r * Math.PI 2 * r * Math.PI 2 r Math.PI Math Math.PI System.out.println(c) System.out.println System.out System System.out c public class Main {
    public static void main(String[] args) {
        Main main = new Main();
        main.run();
    }
    
    // 変数一覧
    double r;

    public void run() {
        // 入力情報取得
        Scanner sc = new Scanner(System.in);
        r = sc.nextInt();
        sc.close();

        // 解答処理
        solve();
    }
    
    private void solve() {
        double c = 2 * r * Math.PI;
        System.out.println(c);
    }
} public class Main {
    public static void main(String[] args) {
        Main main = new Main();
        main.run();
    }
    
    // 変数一覧
    double r;

    public void run() {
        // 入力情報取得
        Scanner sc = new Scanner(System.in);
        r = sc.nextInt();
        sc.close();

        // 解答処理
        solve();
    }
    
    private void solve() {
        double c = 2 * r * Math.PI;
        System.out.println(c);
    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) {
        Main main = new Main();
        main.run();
    }
    
    // 変数一覧
    double r;

    public void run() {
        // 入力情報取得
        Scanner sc = new Scanner(System.in);
        r = sc.nextInt();
        sc.close();

        // 解答処理
        solve();
    }
    
    private void solve() {
        double c = 2 * r * Math.PI;
        System.out.println(c);
    }
}