#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0), cout.tie(0);
  int n;
  cin >> n;
  double answer = 2 * n * 3.1415926535897932384626;
  cout << setprecision(20) << answer << "\n";
  return 0;
}
