import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Scanner;

public class Test {

    public static void main(String[] args) {
        final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
        int r = in.nextInt();
        System.out.println(2.0*Math.PI*r);
    }
}
