#include<bits/stdc++.h>
using namespace std;
static int fast_io = []() { std::ios::sync_with_stdio(false); cin.tie(nullptr); return 0; }();
# define M_PI           3.14159265358979323846  /* pi */
int main(){
  double r;
  cin>>r;
  cout<<M_PI*2*r<<"\n";  


  return 0;
}
