#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  double r;
  cin >> r;
  double pi = atan(1) * 4;
  cerr << pi << endl;
  cout << setprecision(10) << 2 * pi * r << endl;
  return 0;
}
