//#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (n); i++)
#define all(v) v.begin(), v.end()
#define allr(v) v.rbegin(), v.rend()



int main() {
  double r;
  cin >> r;
  cout << 2*M_PI*r << endl;
  
}