#include <bits/stdc++.h>
using namespace std;
const int MOD = 1000000007;
template <class T>
bool chmax(T& a, const T& b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
bool chmin(T& a, const T& b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
int main() {
  double R;
  cin >> R;
  double ans = 2 * R * M_PI;
  cout << std::fixed << std::setprecision(15) << ans << endl;
}
