#include <bits/stdc++.h>
using namespace std;
void fast() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
}
int32_t main() {
  fast();
  double n;
  cin >> n;
  cout << 6.28318530717958623200 * n << "\n";
  return 0;
}
