#include <bits/stdc++.h>
using namespace std;
inline void logger() { std::cout << " [LOGGER] " << endl; }
template <typename A, typename... B>
void logger(const A& a, const B&... b) {
  cout << a << " , ";
  logger(b...);
}
const long long MOD = 1000000007;
const long long MOD9 = 998244353;
const long long INF = MOD * MOD;
const int MAX = 200005;
int main() {
  int r;
  cin >> r;
  std::cout << fixed << setprecision(10);
  cout << (r * 2 * M_PI) << endl;
}
