#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <typename T = ll>
using v = vector<T>;
template <typename T = ll>
using vv = v<v<T>>;
template <typename T = ll, typename U = ll>
using p = pair<T, U>;
template <typename T>
T chmin(T &a, T b) {
  return a = min(a, b);
}
template <typename T>
T chmax(T &a, T b) {
  return a = max(a, b);
}
const ll INF = (ll)1e9;
void a() {
  double r;
  cin >> r;
  cout << fixed << setprecision(15) << 2 * M_PI * r << endl;
}
void b() {}
void c() {}
void d() {}
void e() {}
int main() {
  a();
  return 0;
}
