#include <bits/stdc++.h>
using namespace std;
int main() {
  std::ios::sync_with_stdio(false);
  cin.tie(nullptr);
  double r;
  cin >> r;
  const double PI = 3.14159265;
  cout << std::setprecision(20) << std::fixed;
  cout << 2 * M_PI * r << endl;
  return 0;
}
