/**********************
 * Author: Cloud_     *
 * Date:   19.04.2020 * 
 * Time:   17.53.32   *
 *                    *
***********************/
#include <bits/stdc++.h>
using namespace std;

#define gap " "
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
#define pb push_back
#define mp make_pair
#define ff first
#define ss second
#define ok cout<<"ok"<<endl
#define dbg(x) cout<<(#x)<<" is "<<x<<endl
#define fastio {ios_base::sync_with_stdio(false);cin.tie(NULL);}

typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;

const int N=100009;

int main()
{
  //fastio; 
  double pi=2*acos(0.0),ans;
  int r;
  cin>>r;
  ans=2*pi*r;
  printf("%0.9lf\n", ans);
  return 0;
}