
import java.util.*;

public class Main {
    void run() {
        Scanner sc = new Scanner(System.in);
        int r = sc.nextInt();
        System.out.println(r * 2 * Math.PI);
    }

    void debug(Object... os) {
        System.err.println(Arrays.deepToString(os));
    }

    public static void main(String[] args) {
        new Main().run();
    }
} import java.util.*; util.* java public class Main {
    void run() {
        Scanner sc = new Scanner(System.in);
        int r = sc.nextInt();
        System.out.println(r * 2 * Math.PI);
    }

    void debug(Object... os) {
        System.err.println(Arrays.deepToString(os));
    }

    public static void main(String[] args) {
        new Main().run();
    }
} Main void run() {
        Scanner sc = new Scanner(System.in);
        int r = sc.nextInt();
        System.out.println(r * 2 * Math.PI);
    } run {
        Scanner sc = new Scanner(System.in);
        int r = sc.nextInt();
        System.out.println(r * 2 * Math.PI);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int r = sc.nextInt(); r sc.nextInt() sc.nextInt sc System.out.println(r * 2 * Math.PI) System.out.println System.out System System.out r * 2 * Math.PI r * 2 * Math.PI r 2 Math.PI Math Math.PI void debug(Object... os) {
        System.err.println(Arrays.deepToString(os));
    } debug {
        System.err.println(Arrays.deepToString(os));
    } System.err.println(Arrays.deepToString(os)) System.err.println System.err System System.err Arrays.deepToString(os) Arrays.deepToString Arrays os Object... os os public static void main(String[] args) {
        new Main().run();
    } main {
        new Main().run();
    } new Main().run() new Main().run new Main() String[] args args public class Main {
    void run() {
        Scanner sc = new Scanner(System.in);
        int r = sc.nextInt();
        System.out.println(r * 2 * Math.PI);
    }

    void debug(Object... os) {
        System.err.println(Arrays.deepToString(os));
    }

    public static void main(String[] args) {
        new Main().run();
    }
} public class Main {
    void run() {
        Scanner sc = new Scanner(System.in);
        int r = sc.nextInt();
        System.out.println(r * 2 * Math.PI);
    }

    void debug(Object... os) {
        System.err.println(Arrays.deepToString(os));
    }

    public static void main(String[] args) {
        new Main().run();
    }
} Main

import java.util.*;

public class Main {
    void run() {
        Scanner sc = new Scanner(System.in);
        int r = sc.nextInt();
        System.out.println(r * 2 * Math.PI);
    }

    void debug(Object... os) {
        System.err.println(Arrays.deepToString(os));
    }

    public static void main(String[] args) {
        new Main().run();
    }
}
