#include <bits/stdc++.h>
using namespace std;
inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T>
inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}
int main() {
  std::ios::sync_with_stdio(false);
  int r;
  cin >> r;
  cout << 2 * r * 3.1415926535 << endl;
}
