#include <bits/stdc++.h>
using namespace std;
using VI = vector<int>;
using VS = vector<string>;
using graph = vector<vector<int>>;
int main() {
  int a;
  cin >> a;
  double pi = 3.14159266358979, ans = a * 2 * pi;
  cout << fixed << setprecision(10) << ans << endl;
  return 0;
}
