import java.util.*;

class Main{
    public static void main(String args[]){
        Scanner S = new Scanner(System.in);
        int R = Integer.parseInt(S.next());
        System.out.println(2*R*Math.PI);
    }
} import java.util.*; util.* java class Main{
    public static void main(String args[]){
        Scanner S = new Scanner(System.in);
        int R = Integer.parseInt(S.next());
        System.out.println(2*R*Math.PI);
    }
} Main public static void main(String args[]){
        Scanner S = new Scanner(System.in);
        int R = Integer.parseInt(S.next());
        System.out.println(2*R*Math.PI);
    } main {
        Scanner S = new Scanner(System.in);
        int R = Integer.parseInt(S.next());
        System.out.println(2*R*Math.PI);
    } Scanner S = new Scanner(System.in); S new Scanner(System.in) int R = Integer.parseInt(S.next()); R Integer.parseInt(S.next()) Integer.parseInt Integer S.next() S.next S System.out.println(2*R*Math.PI) System.out.println System.out System System.out 2*R*Math.PI 2*R*Math.PI 2 R Math.PI Math Math.PI String args[] args
import java.util.*;

class Main{
    public static void main(String args[]){
        Scanner S = new Scanner(System.in);
        int R = Integer.parseInt(S.next());
        System.out.println(2*R*Math.PI);
    }
}
