#include <bits/stdc++.h>
using namespace std;
const double pi = 3.141592653589793238;
int main() {
  int n, i, j, t;
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n;
  double d = 2 * pi * n;
  cout << fixed << setprecision(9) << d;
  return 0;
}
