#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <typename T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return 0;
}
template <typename T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return 0;
}
const double PI = 3.14159265358979323846;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  double R;
  cin >> R;
  cout << fixed << setprecision(10) << 2 * PI * R << endl;
  return 0;
}
