import java.util.*;
public class Main{
    public static void main(String [] args){
        Scanner sc = new Scanner(System.in);
        int r = sc.nextInt();
        //int b = sc.nextInt();
        //int c = sc.nextInt();
        //String s = sc.next();

/*        if(N%2 != 0){
        for(int i = 1; i <= N; i++){
            for(int j = i; j <= N; j++){
                c = i*j;
                if(c == N){
                    d = i+j-2;
                    if(min == 0 || min > d) min = d;
            }
        }
    }
}*/
double R = (double)r * Math.PI * 2;
    System.out.println(R);

    }
}