import java.util.*;
import java.math.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sn = new Scanner(System.in);
        
        int r = sn.nextInt();
        BigDecimal num = BigDecimal.valueOf(r * 6.28);
        
        System.out.println(num);

    }
}

/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