#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);
  double r, pi = 3.14159265358979323;
  cin >> r;
  double ans = 2 * r;
  ans *= pi;
  cout << fixed << setprecision(12) << ans << '\n';
  return 0;
}
