#include <bits/stdc++.h>
using namespace std;
long long p = 748683265;
int main() {
  long long r;
  cin >> r;
  double l = 2 * (3.41592653589793238462);
  double z = l * r;
  cout << fixed << setprecision(21) << z << endl;
  return 0;
}
