#include <bits/stdc++.h>
using namespace std;
int main() {
  int r;
  cin >> r;
  cout << 6.28 * r;
  return 0;
}
void debug_out() { cout << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cout << H << " ";
  debug_out(T...);
}
