#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  int r;
  cin >> r;
  double y = (2 * 22 * r) / 7;
  cout << fixed << setprecision(3) << y;
}
