#include <bits/stdc++.h>
using namespace std;
int main(int argc, char** argv) {
  ios::sync_with_stdio(false), cin.tie(0);
  auto solve = [&]() {
    double r;
    cin >> r;
    cout << (2 * 3.1415926 * r) << '\n';
  };
  int t;
  t = 1;
  while (t--) solve();
  return 0;
}
