import java.util.Scanner;

class Main {
    static final int MOD = 1000000007;
    static final int INF = 1 << 30;
    static final int ALF = 26;

    Main() {
        Scanner sc = new Scanner(System.in);
        double R = sc.nextInt();
        System.out.println(Math.PI*R*2);
    }

    public static void main(String[] args) {
        new Main();
    }
} import java.util.Scanner; Scanner java.util class Main {
    static final int MOD = 1000000007;
    static final int INF = 1 << 30;
    static final int ALF = 26;

    Main() {
        Scanner sc = new Scanner(System.in);
        double R = sc.nextInt();
        System.out.println(Math.PI*R*2);
    }

    public static void main(String[] args) {
        new Main();
    }
} Main static final int MOD = 1000000007; MOD 1000000007 static final int INF = 1 << 30; INF 1 << 30 1 30 static final int ALF = 26; ALF 26 Main() {
        Scanner sc = new Scanner(System.in);
        double R = sc.nextInt();
        System.out.println(Math.PI*R*2);
    } Main {
        Scanner sc = new Scanner(System.in);
        double R = sc.nextInt();
        System.out.println(Math.PI*R*2);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double R = sc.nextInt(); R sc.nextInt() sc.nextInt sc System.out.println(Math.PI*R*2) System.out.println System.out System System.out Math.PI*R*2 Math.PI*R*2 Math.PI Math Math.PI R 2 public static void main(String[] args) {
        new Main();
    } main {
        new Main();
    } new Main() String[] args args
import java.util.Scanner;

class Main {
    static final int MOD = 1000000007;
    static final int INF = 1 << 30;
    static final int ALF = 26;

    Main() {
        Scanner sc = new Scanner(System.in);
        double R = sc.nextInt();
        System.out.println(Math.PI*R*2);
    }

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