#include <bits/stdc++.h>
#include <map> // pair
using namespace std;

typedef long long ll;
#define vec vector<int>
#define vecll vector<ll>
#define vpair vector<pair<int, int>>
#define vpairll vector<pair<ll, ll>>
#define rep(i,n)   for(int i=(int)0; i<(int)n; i++)
#define llrep(i,n)   for(ll i=(ll)0; i<(ll)n; i++)
#define REP(i,m,n) for(int i=(int)m; i<(int)n; i++)
#define all(var)     (var).begin(), (var).end()
#define prt(var)     cout << var << endl
#define prt_dbl(i,var) cout<<fixed<<setprecision(i)<<var<<endl;
template <typename Val>
Val gcd(Val a, Val b) {
//use ll or int for gcd
    if (b==0) return a;
    else return gcd(b, a%b);
}
template <typename Val>
Val vec_max(vector<Val> v){
  return *max_element(v.begin(),v.end());
}
template <typename Val>
Val vec_min(vector<Val> v){
  return *min_element(v.begin(),v.end());
}
//---------------------------------------------------------------
int main(){
  double N;
  cin>>N;
  double pi =  3.14159265358979323846;
  N = 2*N*pi;
  prt_dbl(4,N)

}
