#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
template <typename T, typename S>
ostream& operator<<(ostream& os, pair<T, S> a) {
  os << '(' << a.first << ',' << a.second << ')';
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, vector<T> v) {
  for (auto x : v) os << x << ' ';
  return os;
}
void debug() { cerr << '\n'; }
template <typename H, typename... T>
void debug(H a, T... b) {
  cerr << a;
  if (sizeof...(b)) cerr << ", ";
  debug(b...);
}
int main() {
  double r;
  cin >> r;
  printf("%.10Lf\n", 2 * r * M_PI);
}
