#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long double r;
  cin >> r;
  long double w = 2.0 * M_PI * r;
  cout << setprecision(10) << fixed << w;
}
