#include<iostream>
#include<cmath>
#include<algorithm>
#include <iomanip> 
#include<vector>
#define fo(i,n) for(ll i=0;i<n;i++)
#define f(i,n) for(ll i=1;i<n;i++)
#define ff(i,k,n) for(ll i=k,i<n;i++)
#define ll long long
/*Elegizip, elegizip otken sol bir shaktar-ai
 Elendeter zamanymnyn otip bara zhatkanny-ai
Umytylmas-Umytylmas Kairan esil shaktar-ai
Zamanymnyn kolyn bulgap otip bara jatkany-ai
 kairan esil , kairan esil shaktar-ai
kolyn bulgap zamanymnyn otip bara jatkany-ai;*/
using namespace std;

int main()
{ios::sync_with_stdio(0);
double n, r=0;
cin>>n;
cout<<2*3.14*n;

}



