#include <bits/stdc++.h>
using namespace std;
struct second_order {
  bool operator()(const pair<string, int>& x,
                  const pair<string, int>& y) const {
    if (x.second != y.second) return x.second < y.second;
    return x.first > y.first;
  }
};
typedef priority_queue<pair<string, int>, vector<pair<string, int> >,
                       second_order>
    pqueue_type;
pqueue_type map_to_pqueue(map<string, int> m) {
  pqueue_type que;
  for (auto it = m.begin(); it != m.end(); ++it) {
    que.push(*it);
  }
  return que;
}
int main(void) {
  int n;
  cin >> n;
  cout << 3.1415926535897 * n * 2 << endl;
  return 0;
}
