#include <bits/stdc++.h>
using namespace std;
const int INF = 2000000000;
void CINT() {}
template <class Head, class... Tail>
void CINT(Head&& head, Tail&&... tail) {
  cin >> head;
  CINT(move(tail)...);
}
void Main() {
  long long r;
  cin >> r;
  cout << r * 2 * M_PI;
  cout << endl;
}
int main() {
  std::cout << std::fixed << std::setprecision(15);
  Main();
}
