#include <bits/stdc++.h>
using namespace std;
template <typename T>
inline istream& operator>>(istream& i, vector<T>& v) {
  for (int j = 0; j < ((int)(v).size()); ++j) i >> v[j];
  return i;
}
template <typename T1, typename T2>
inline istream& operator>>(istream& i, pair<T1, T2>& v) {
  return i >> v.first >> v.second;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
long long INF = 1001001001;
long long LINF = 1001001001001001001ll;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  double r;
  cin >> r;
  printf("%.10f\n", r * 2 * acos(-1));
}
