#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
constexpr ll LLINF{1001002003004005006};
constexpr int INTINF{1000000000};
template <typename T>
void maxs(T& x, T&& y) {
  x = std::max(x, y);
}
template <typename T>
void maxs(T& x, T& y) {
  x = std::max(x, y);
}
template <typename T>
void mins(T& x, T&& y) {
  x = std::min(x, y);
}
template <typename T>
void mins(T& x, T& y) {
  x = std::min(x, y);
}
int main() {
  ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  int A;
  cin >> A;
  cout << (3.14159265 * A * 2) << '\n';
}
