import java.util.*;
import java.math.*;

public class Main {
    static final int MOD = 1_000_000_007; // 10^9+7
    // static final int MAX = 2_147_483_646; // intの最大値
    static final int INF = 1_000_000_000; // 10^9
    static final int MAX = 10_000_000;

    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        double n = sc.nextDouble();
        System.out.println(2*n*Math.PI);
        

    }

} import java.util.*; util.* java import java.math.*; math.* java public class Main {
    static final int MOD = 1_000_000_007; // 10^9+7
    // static final int MAX = 2_147_483_646; // intの最大値
    static final int INF = 1_000_000_000; // 10^9
    static final int MAX = 10_000_000;

    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        double n = sc.nextDouble();
        System.out.println(2*n*Math.PI);
        

    }

} Main static final int MOD = 1_000_000_007; // 10^9+7 MOD 1_000_000_007 // static final int MAX = 2_147_483_646; // intの最大値
    static final int INF = 1_000_000_000; // 10^9 INF 1_000_000_000 static final int MAX = 10_000_000; MAX 10_000_000 public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        double n = sc.nextDouble();
        System.out.println(2*n*Math.PI);
        

    } main {
        Scanner sc = new Scanner(System.in);
        double n = sc.nextDouble();
        System.out.println(2*n*Math.PI);
        

    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double n = sc.nextDouble(); n sc.nextDouble() sc.nextDouble sc System.out.println(2*n*Math.PI) System.out.println System.out System System.out 2*n*Math.PI 2*n*Math.PI 2 n Math.PI Math Math.PI String args[] args public class Main {
    static final int MOD = 1_000_000_007; // 10^9+7
    // static final int MAX = 2_147_483_646; // intの最大値
    static final int INF = 1_000_000_000; // 10^9
    static final int MAX = 10_000_000;

    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        double n = sc.nextDouble();
        System.out.println(2*n*Math.PI);
        

    }

} public class Main {
    static final int MOD = 1_000_000_007; // 10^9+7
    // static final int MAX = 2_147_483_646; // intの最大値
    static final int INF = 1_000_000_000; // 10^9
    static final int MAX = 10_000_000;

    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        double n = sc.nextDouble();
        System.out.println(2*n*Math.PI);
        

    }

} Main
import java.util.*;
import java.math.*;

public class Main {
    static final int MOD = 1_000_000_007; // 10^9+7
    // static final int MAX = 2_147_483_646; // intの最大値
    static final int INF = 1_000_000_000; // 10^9
    static final int MAX = 10_000_000;

    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        double n = sc.nextDouble();
        System.out.println(2*n*Math.PI);
        

    }

}
