#include <bits/stdc++.h>
using namespace std;
double solve(int64_t R) { cout << (long double)R * M_PI * 2 << endl; }
int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  constexpr char endl = '\n';
  int64_t R;
  cin >> R;
  auto ans = solve(R);
  cout << ans << endl;
  return 0;
}
