#include <bits/stdc++.h>
using namespace std;
template <class T>
inline void chmax(T& a, T b) {
  if (a < b) a = b;
}
template <class T>
inline void chmin(T& a, T b) {
  if (a > b) a = b;
}
const long long INF = 1LL << 60;
int main() {
  int r;
  cin >> r;
  const double pi = 3.1415926535;
  double leng = 2 * pi * r;
  cout << leng << endl;
  return 0;
}
