#include <bits/stdc++.h>
using namespace std;
template <typename T1, typename T2>
inline void chmin(T1 &a, T2 b) {
  if (a > b) a = b;
}
template <typename T1, typename T2>
inline void chmax(T1 &a, T2 b) {
  if (a < b) a = b;
}
signed main() {
  double r;
  cin >> r;
  double num = r * 3.1415926 * 2;
  cout << fixed << setprecision(10) << num << endl;
}
