#include <bits/stdc++.h>
using namespace std;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vs = vector<string>;
using ll = long long;
template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec) is >> x;
  return is;
}
template <typename T>
ostream &operator<<(ostream &os, vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i] << (i + 1 == vec.size() ? "" : ' ');
  }
  return os;
}
int main() {
  int r;
  cin >> r;
  double ans = 3.141592648777698869248;
  printf("%.10lf\n", r * 2 * ans);
}
