#include <bits/stdc++.h>
#define fo(i, n) for(int i = 0; i < n; ++i)
#define ll long long int
#define fok(i, k, n) for(int i = k; i < n; ++i)
#define MOD 1000000000+7
#define all(X) X.begin(), X.end()
#define pb push_back
#define present(a, x) (a.find(x)!=a.end())
using namespace std;

map <int, set <int> > m;

int main(){
  double stime = clock();
  double r;
  cin>>r;
  cout<<fixed<<setprecision(12)<<(2*3.14159265*r)<<'\n';


  double etime = clock();
  
   // cerr << "Execution time: " << (etime - stime) / CLOCKS_PER_SEC * 1000 << " ms.\n";
}