#include <bits/stdc++.h>
using namespace std;
long double pi = acos(-1);
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long double n;
  cin >> n;
  cout << fixed << setprecision(9) << (long double)2 * n * pi << '\n';
  return 0;
}
