#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
  long double pair<int, int> = acos(-1);
  long double r;
  cin >> r;
  cout << r * 2.0 * pair<int, int> << endl;
}
