#include <bits/stdc++.h>
using namespace std;
int main() {
  freopen("in.txt", "r", stdin);
  int r;
  cin >> r;
  double c;
  c = 2 * 3.1415926535897932384626433 * r;
  cout << setprecision(30) << c << endl;
  return 0;
}
