#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  double n;
  cin >> n;
  double ans = 2 * 3.141592653589793238 * n;
  cout << fixed << setprecision(20) << ans << endl;
  return 0;
}
