#include <bits/stdc++.h>
using namespace std;
const long long INF = 1LL << 60;
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
int main() {
  int r;
  double pi = 3.14159265;
  cin >> r;
  cout << setprecision(20) << double(2 * pi * r) << endl;
  return 0;
}
