/*
 * @Author: Luisvacson
 * @LastEditors: Luisvacson
 * @Descriptions: None
 * @Date: 2020-04-19 20:43:14
 * @LastEditTime: 2020-04-19 20:46:28
 * @FilePath: \VSC\C++\Practice\AC abc163 A.cpp
 */
#include<bits/stdc++.h>
using namespace std;
#define Pi 3.14159265358979323846

signed main(){
    long double R;
    cin>>R;
    cout<<Pi*R*2<<'\n';

    return 0;
}