#include <bits/stdc++.h>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#define ll long long
vector<int> 
using namespace std; 
signed
main()
{
double r;
cin>>r;
cout<<setprecision(9)<<2*3.141592653589793238*r;



		
}
