#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

int main(){
  
  double pi = 3.1415926535;
  int R;
  cin >> R;
  float cir = 2 * pi * R;
  cout << cir << endl;
  
  return 0;
}