#include <vector>
#include <iostream>
#include <string>
#include <math.h>
#include <algorithm>
#include <bits/stdc++.h>


using namespace std;


int main(void){
  
  int R;
  double ans=0;
  
  
  cin >> R;

  ans = 2 * M_PI*R;

  cout << ans << endl;
  

  return 0;
}


