//
//  main.cpp
//  HelloWorld
//
//  Created by purple on 2020/04/13.
//  Copyright © 2020 Purple. All rights reserved.
//

#include <iostream>
#include <algorithm>
#include <string>
#include <fstream>
#include <regex>

using namespace std;

int main(int argc, const char * argv[]) {
    // insert code here...
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    #define PI 3.14159265359


    
    int r;
    
    cin >> r;
    
    cout << 2*r*PI;
    
    return 0;
    
}
