#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const long long INF = 1LL << 60;
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const double PI = 3.14159265358979323846;
int main() {
  double R;
  cin >> R;
  cout << 2 * R * PI << endl;
}
