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

public class Main {

	public static void main(String[] args) throws IOException{
		// TODO Auto-generated method stub
		BufferedReader bfr= new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer str = new StringTokenizer(bfr.readLine());
		int R = Integer.parseInt(str.nextToken());
		double answer = 2*(3.141592)*R;
		System.out.println(answer);
	}

}

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz