#include <bits/stdc++.h>
using namespace std;
template <class T>
inline void chmax(T &ans, T t) {
  if (t > ans) ans = t;
}
template <class T>
inline void chmin(T &ans, T t) {
  if (t < ans) ans = t;
}
int main() {
  double r;
  cin >> r;
  cout << setprecision(11) << fixed;
  cout << r * 2.0 * 3.1415926 << endl;
}
