#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  double res;
  int value;
  cin >> value;
  res = (double)2 * value * (double)M_PI;
  cout << setprecision(10) << fixed << res << endl;
  return 0;
}
