#include<bits/stdc++.h>
using namespace std;
#define Endl '\n'
typedef long long int lli;
/*
created on: 19/4/2020
*/
void solve();
int main(int argc, char const *argv[]){
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  solve();
}
void solve(){
  double R;
  cin>>R;
  double pie = 6.28318530717958623200/2;
  printf("%.8f\n", 2*pie*R);
}
