#include <bits/stdc++.h>
#include<queue>
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
typedef pair<ll,ll> Pll;
typedef pair<string,string> Pstring;
typedef pair<double,double> Pdouble;

#define REP(i,n) for(ll i=0;i<n;i++)
#define REP1(i,n) for(ll i=1;i<=n;i++)
#define Precision13 cout << fixed << setprecision(13)
ll MOD = 1e9+7;
const double PI=3.14159265358979323846;


int main() {
  ll r;
  cin >> r;
  Precision13;
  cout << r*2*PI<<endl;
}