#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i=0; i<(n); ++i)
#define REP(i, d, n) for(int i=(d); i<(n); ++i)
#define all(v) v.begin(), v.end()
using ll = long long;
int main() {

int r; cin>>r;
  int rr;
  rr=2*r;
  
  double ans;
  ans=rr*3.1415926535;
  cout<<ans<<endl;
  

}