#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 9;
const double pi = acos(-1);
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  double r;
  cin >> r;
  cout << fixed << setprecision(20) << 2 * pi * r << '\n';
  return 0;
}
