//package AtCoder_163;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class ProblemA {
    public static void main(String[] args)throws IOException {
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int r=Integer.parseInt(br.readLine());
        System.out.println(2*Math.PI*r);
    }
}
