#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cout.tie(nullptr);
  cin.tie(nullptr);
  int r;
  cin >> r;
  double cir = 2 * M_PI * (double)r;
  cout << fixed << setprecision(10) << cir;
}
