#include <bits/stdc++.h>
using namespace std;
const int A = 5e2 + 2;
const int mod = 1e9 + 7;
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  double r;
  cin >> r;
  cout << setprecision(6) << fixed;
  cout << 2.0 * acos(0) * r;
  return 0;
}
