/******************************************************************************

                              Online C++ Compiler.
               Code, Compile, Run and Debug C++ program online.
Write your code in this editor and press "Run" button to compile and execute it.

*******************************************************************************/

#include <bits/stdc++.h>

using namespace std;

int main()
{
    int r;
    cin>>r;
    
    double pi = 3.14159265358979323846;
    cout<<setprecision(9)<<pi*2*r;
    
}
