#include <bits/stdc++.h>
using namespace std;
template <typename T>
void printlnVector(T v) {
  for (int i = 0; i < (int)(v.size()); i++) {
    cout << v[i] << endl;
  }
}
const double PI = 3.14159265358979323846;
template <typename T>
void printVector(T v) {
  for (int i = 0; i < (int)(v.size()); i++) {
    cout << v[i] << " ";
  }
  cout << endl;
}
int main() {
  int R;
  cin >> R;
  long double ans = 2 * R * PI;
  cout << fixed << setprecision(15) << ans << endl;
  return 0;
}
