#include <bits/stdc++.h>
using namespace std;
void solve() {
  int r;
  static const double PI = acos(-1);
  cin >> r;
  cout << 2 * PI * r << "\n";
}
int main(int argc, char *argv[]) {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int o_o = 1;
  if (!o_o) cin >> o_o;
  for (; o_o; --o_o) solve();
  return 0;
}
