#include <bits/stdc++.h>
using namespace std;
template <typename T>
using lim = numeric_limits<T>;
const double PI = acos(-1);
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  double r;
  cin >> r;
  cout << r * 2 * PI << "\n";
}
