#include <bits/stdc++.h>
using namespace std;
using ll = long long;
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;
}
const int inf = 1000000000;
int main() {
  int R;
  cin >> R;
  const double PI = 3.14159265358979323846;
  printf("%.16f", R * 2 * PI);
  return 0;
}
